■J*t.
Digitized by tine Internet Arciiive
in 2007 with funding from
IVIicrosoft Corporation
Iittp://www.arcliive.org/details/conversationsonc01ostwrich
CONVERSATIONS ON CHEMISTRY.
FIRST STEPS IN CHEMISTRY.
Part I.
General Chemistry.
BY
Prof. W. OSTWALD.
AUTHORIZED TRANSLATION
BY
ELIZABETH CATHERINE RAMSAY.
i2mo, viii -|- 250 pages, 46 figures. Cloth, $1.50.
CONVERSATIONS ON CHEMISTRY
FIRST STEPS IN CHEMISTET
BY
W. OSTWALD
Professor of Chemistry in the University of Leipzig
ATTTHOmZED TRANSLATION
BY
ELIZABETH CATHERINE RAMSAY
Part I
GENERAL CHEMISTRY
FIRST EDITION, CORRECTED
THIRD THOUSAND
NEW YORK
JOHN WILEY & SONS
London: CHAPMAN & HALL, Limited
1911
Copyriglit, 1905,
BY
ELIZABETH CATHERINE RAMSAY.
Entered at Stationers' Hall.
THF SCIENTIFIC PRESS
nOBERT ORUMMOND AND COMPANY
BROOKLYN, N. Y.
-. .vA^.*iA*.'W» c. ■ . i^-V.
AUTHOR'S PREFACE.
The causes which led me to write this work lie partly
in the past, partly in the future. The former spring
from the feeling of thankfulness with which I even now
regard the "Schule der Chemie" of Stockhardt, whose
memory still lingers among us. By a stroke of good
fortune this excellent work was the first text-book of
chemistry which was placed in my hands, and it influ-
enced the whole of my subsequent activity in science.
Owing to the carefully thought-out directness in repre-
senting the facts to the pupil, the skill in selecting experi-
ments suitable to the physical and mental powers of
the beginner, I have never lost touch with experiment,
although I have been chiefly occupied with general
questions of science. The request of the publishers, who
used to issue this work, that I should write a modern
Stockhardt, was both an honour and an opportunity of
paying off an old debt of thankfulness.
So much for the past.
As regards the future, chemistry has undergone during
the past century an enormous development, in which Ger-
many has played an important part. Chemical science in
Germany has been furthered by the work of thousands
of diHgent hands and greatly aided by educational insti-
tutions which have become a pattern for the whole world,
which have brought about a constant interchange between
IV AUTHOR'S PREFACE.
science and its applications, and which have given an
uninterrupted proof of a continued heahhy existence.
It was almost entirely organic chemistry which developed
in the direction of the discovery of new bodies and their
systematic arrangement ; and even to this day, by far the
majority of young chemists, after hurrying through a short
course of analysis, are trained in these methods.
But hasty progress has its dangers, and it is the duty
of every man who tries to look into the future to give
a timely word of warning; for inorganic chemistry was
a science before organic chemistry was thought of; more-
over, the processes of inorganic chemistry form the basis
of chemical technology, on which that of organic com-
pounds is a superstructure.
The cry was first raised in manufacturing circles that
the young chemist trained exclusively in organic chem-
istry was unfit to cope with the solution of general prob-
lems; with that reciprocity between science and tech-
nology so characteristic of the German race, the teachers
of our science have at once grappled with the problem.
Among the many proposals which have been made to
escape, in good time, the pressing danger of chemical
onesidedness, none appears to me more suitable than
the encouragement of the growth which has developed
upon the soil of science during the last ten years. I refer
to general and physical chemistry. It deals with ques-
tions which he at the base of organic and inorganic, of
pure and applied chemistry; it forms a foundation for
all real chemical education, and must be regarded as
lying at the root of all chemical teaching, especially in
its earlier stages.
By writing a series of text-books dealing with different
stages of the subject I have tried to bring about the
AUTHOR'S PREFACE. V
knowledge of these principles as they at present exist,
first among my colleagues in science, and next among
students of chemistry.
The necessity of repeatedly revising the matter of
these books, as well as daily experience in teaching,
led to my early conviction that the very first steps of a
young pupil must point in this direction; I also gained
assurance that such an introduction was possible, and
this book is the result of my efforts.
I must not omit to mention that it forms the first
introductory volume, and that it will be followed as soon
as possible by a second of about equal length, in which
the system will be more developed.
I have chosen the form of dialogue, because after
several attempts it appeared to me the most suitable;
moreover, I have come to the conclusion that it occupies
no more space than an ordinary description, while the
impression it makes is much more penetrating and
lively. I venture to hope that it will be found that it is
at the same time the result of a varied experience in
teaching, and not an accidental choice.
W. OSTWALD.
Leipzig, 1903.
CONTENTS,
PAGB
1. Substances i
2. Properties 5
3. Substances and Mixtures 10
4. Solutions 16
5. Melting and Freezing 23
6. Boiling and Evaporation 28
7. Measuring 36
8. Density 46
9. Forms 54
10. Combustion 61
11. Oxygen 71
12. Compounds and Constituents 82
13. Elements 92
14. Light Metals 103
15. Heavy Metals 113
16. More about Oxygen 117
17. Hydrogen 129
18. Oxygen and Hydrogen 139
19. Water 152
20. Ice 163
21. Steam 171
22. Nitrogen 182
23. Air 188
24. Continuity and Exactness 200
25. The Expansion of Air by Heat 208
26. The Water in the Air 218
27. Carbon 224
28. Carbon Monoxide 234
29. Carbon Dioxide 237
30. The Sun 244
vii
CONVERSATIONS IN CHEMISTRY.
1. SUBSTANCES.
Master. To-day we commence something quite new;
you shall begin to learn chemistry.
Pupil. What is chemistry ?
M. Chemistry is a branch of natural science. You
have already learned something about animals and plants
and know that the study of animals is called zoology, and
that of plants botany.
P. Then does chemistry teach about stones?
M. No, that is called mineralogy. Mineralogy is not
the study of stones alone, but of many other things which
are found in the earth, such as phosphorus, gold, or coal.
But all these things, too, belong to chemistry. And
other things, like sugar, glass, iron, which are not found
in the earth, but are artificially obtained from other sub-
stances, are also the subjects of chemistry. Chemistry is
the study of all kinds of substances, whether artificial or
natural.
P. Then does chemistry deal with trees ?
M. No, for a tree is not a substance.
P. But it is wood, and wood is a substance.
M. Yes, but a tree consists of more than wood, for its
leaves and fruit are not made of wood, but of other sub-
2 CONyERSATIONS ON CHEMISTRY.
stances. All such substances taken alone belong to
chemistry; but to get each alone, the tree must be de-
stroyed.
P. But what do you mean by a substance ?
M. That is a long story. Let me see if you don't
know it yourself, though you can't put it into words.
What is this?
P. I think it is sugar.
M, Why?
P. Well, the sugar in the sugar-basin looks just like it.
Let me taste it — yes, it's sugar, for it's sweet.
M. Do you know another way by which you can tell
sugar ?
P. Yes, it makes your fingers sticky; so does this.
M. You can tell sugar, then, when some one puts it in
your hand and asks you if it is sugar. And you knew it,
first by its appearance, then by its taste, and lastly by its
stickiness. These signs by which you recognize a sub-
stance are called ''properties"; you know sugar by its
properties. Sugar is a substance; one can tell substances
by their properties. Do you think you could use all the
properties of a substance in order to recognize it ?
P. Yes, if I knew them.
M. We will just see. Is there only one sort of sugar?
No, you know loaf sugar, which is in large lumps, and
sifted sugar, which is a powder, like sand. Both are
sugar, because when you pound up loaf sugar it becomes
like sifted sugar.
P. Yes: then they are both the same.
M. Both are the same substance, sugar, but one of its
properties has been changed. The shape of a thing is
also one of its properties; if you like you can change its
shape, yet the stuff of which it consists remains the same.
SUBSTANCES. 3
This also applies to quantity. Whether the sugar-basin
is full or almost empty, what is in it is always sugar. So
you see form and quantity do not belong to the properties
by which you recognize a substance. Is sugar hot or
cold?
P. I don't know; it may be either.
M. Quite right. So neither heat nor cold is a property
by which you can tell a substance.
P. No, of course you can't; for you can make sugar as
coarse or as fine, or as hot or cold as you wish.
M. Now we have got to the bottom of it. Among the
properties of a thing there are some which cannot be
altered. You will always find that sugar tastes sweet,
and that it makes your fingers sticky. But you can
change its size and form, and you can heat it if you like.
Every definite substance has its distinct unchangeable
properties, and a thing bears the name of this substance
when it has these fixed unchangeable properties, quite
independently of whether it is warm or cold, large or
small, or how its changeable properties may vary.
A thing has often another name, according to its use
or its shape, different from that of the substance it is made
of. Then it is said to consist of this particular substance.
P. I don't quite understand that.
M. What's this? what's that?
P. A knitting-needle and a pair of scissors.
M. Are knitting-needles and scissors substances?
P. I'm not sure — No, I think not.
M. If you wish to know, you have only to ask: What
does the thing consist of, or what is it made of? Then
you generally come at the name of the substance. What
are knitting-needles and scissors made of?
P. Of iron. Then is iron a substance?
4 CONVERSATIONS ON CHEMISTRY,
M, Certainly, for a piece of iron is called iron, whether
it is large or small, hot or cold.
P. Then paper is a substance, because a book is made
of paper, and wood is a substance, because the table is
made of wood, and bricks are a substance, because
houses are made of bricks.
M. The first two examples are right, but not the last.
Does a brick remain a brick when it is broken up and
powdered? No: the name "brick" is given to a thing
that has a definite shape, so it can't be a stuff. But what
are bricks made of?
P. They're made of clay.
M. Is clay a substance ?
P. Yes — no — yes, it is, because if you break up clay
it still remains clay.
M. Quite right. You can often help yourself out like
that when you are in doubt. First you must ask: What
is the thing made of? And when you have answered
that, you must go further, and ask: Does it remain the
same when I break it up? and if you can say Yes, then
it is a substance.
P. Then there are many, many different kinds of sub-
stances?
M. Yes, certainly there are many; far more substances
than you can name. And chemistry has to do with all
such substances.
P. Oh, then I shall never be able to learn all about
chemistry — it's hopeless. I'd rather not begin.
M. Do you know the forest near here ?
P. Yes, rather: you could put me where you like in it,
and I should always know where I was.
M. But you don't know every single tree in it? How
can you help being lost ?
PROPERTIES, S
P. But I know the paths.
M. Now, look here, that is what we are going to do
with chemistry. We will not learn about every single
substance that there is, but we will learn the paths which
divide up the countless substances into different groups,
and by help of which we can find our way from one place
to another.
When you know the principal paths you will know
where you are in chemistry, and afterwards you can leave
the chief paths, and find out more about the byways.
And you will see that learning chemistry is just as good
fun as walking in a wood.
2. PROPERTIES.
M, Let me hear what you learned last time.
P. Chemistry is the study of substances, and sub-
stances are what things consist of.
M. The first part of your answer is right, but the
second is not quite right. A piece of music consists of
sounds; but are sounds substances?
P. Yes; for you can call the sounds music is made of,
substances.
M. Yes, in a figurative sense you can. But in the
language of science the name "substance" is limited
to things that have weight.
P. What right has any one to limit the meaning of a
name?
M. The right of necessity. In the language of ordinary
life people are not generally very careful of the meaning of
words, as you showed yourself just ngw. In science,
however, we have to try to be as accurate as we can,
6 COhiyERSATlONS ON CHEMISTRY.
and that is why we have to give every- day words an
exact and accurate meaning. These meanings are made
as like as possible to those which they ordinarily have,
and really mean the same thing to all intents and pur-
poses, only the boundary-line of use and meaning is
more sharply drawn.
Most things which are generally known as substances
are called the same in chemistry; but no things that
have no weight. Now correct the last part of your sen-
tence: "Substances are everything" . . .
P. A substance is anything of which a weighable
thing consists. Yes, but I don't know yet what a sub-
stance really is.
M. What do you mean?
P. I know quite well what things to call substances,
but that isn't all. It doesn't tell me any more than I
knew before. I know nothing about the nature of a
substance yet.
M. How should you know it? By giving a word a
distinct scientific use, or defining a word, nothing more
has happened than that I drew a circle round it so as
to limit the meaning of the word within certain bounds.
We have made a fence round our forest; but, of course,
that doesn't teach us to know it. As you learn the prop-
erties of the various substances, you will also learn their
nature, and that will give you plenty to do.
P. But when I know all the properties of a substance,
I'll only know — how can I put it? — the outside of it.
I can't get through to its inner nature.
M. Don't you remember that there are different
sorts of properties? What are they?
P. You mean what we spoke of yesterday ? There are
changeable and unchangeable properties.
PROPERTIES. 7
M. And which help you to recognize the substance?
P. The unchangeable ones.
M. There now, you've found what you want. The
unchangeable properties of a substance can't be taken
away; when they aren't there, the substance isn't there
either. These properties make the nature of the substance.
P. But that is only its properties. What I want to
know is : What lies at the bottom of all its properties ?
M. You want to know what remains when you think
all the properties of a substance are taken away. Now,
just think, if you took away all the properties of a piece
of sugar, its colour, form, hardness, weight, taste, etc.,
what would remain?
P. I don't know.
M. Nothing would remain. Because it is only by
the properties I can tell that something is there; if no
properties are present, there is nothing there that I can
speak about. You must get rid of the idea that there
is anything higher or more real to be found in a thing
than its properties. Long ago, when science was little
advanced, people thought something like that, and there
are remains of it in ordinary speech, so that one uncon-
sciously gets these ideas through the use of ordinary
expressions. But once you recognize this error you can
avoid it.
P. I see you are quite right, but I'm afraid it will take
me a long time to get rid of the other idea.
M. You will be convinced when you have barned more
chemistry that we really only speak of the properties
of a stuff, and never of its ''nature.'' And you will
forget your mistake later. — Anyhow, this talk has had
its use, for now you see clearly that everything depends
on our determining and knowing properties. Tell me
8 CONyERSATIONS ON CHEMISTRY,
some properties which help you to recognize a sub-
stance. For example, what is the difference between
silver, gold, and copper?
P. The colour; silver is white, gold yellow, and
copper red.
M. Does colour belong to the changeable or to the
unchangeable properties of a substance?
P. Generally to the unchangeable, I should think.
M. Why are you so uncertain about it?
P. I am not quite sure: the colours of gold and silver
are unchangeable, but old copper doesn't look red, but
dark, and often green.
M. Have you ever looked carefully at a piece of copper
that has become green ? Is the copper green through and
through ?
P. I think not; no, you can scratch off the green, and
there is red copper underneath.
M. Quite right; and the green is, in other ways, not
like copper; it is not tough like metal, but crumbly like
earth. The fact is that another substance, green in colour,
has been found on the copper, that was not there before,
and it has only covered up the red copper, just as the
yellow wood of the window-frame is covered with white
paint.
P. How does the green come on the copper?
M. It is formed from the copper : you will learn later
exactly how it comes. At first we will go back to the
question of colours. Now we must take colour as an
unchangeable property, by which we can recognize a
substance. Only we must take care not to mistake the
colour of a chance layer on the surface for the colour of
the substance itself. We see that best if we break it
into pieces, and so expose the inner part. Let us try it.
PROPERTIES. 9
Look what I have here. It is a blue substance, which
is called copper sulphate.
P. Oh, please don't break it up, it has such a lovely
shape, just Hke a cut jewel.
M. Those shapes are called crystals; they are not
made by cutting, but form themselves without our help.
P. May I see that?
M, You will soon learn for yourself how to form
crystals. I have a great many more, and we can quite
well use this one, if we are going to learn anything by it.
There, I have broken it: look closely if the blue coloui
of this stuff is its own.
P. Yes, it is, because the stuff is just as bright a blue
inside as outside.
M. Now we will break it up still smaller in this thick
little porcelain dish, which is
called a mortar. For that we
will use this thick rod, which
is called a pestle (Fig. i).
P. Why are you giving your-
self so much unnecessary
bother? We know already ^^^- ^•
what will happen.
M. Look at it carefully. When you have drawn a con-
clusion you must test it properly, or else you won't know
that you haven't overlooked or forgotten something.
What do you see ?
P. The pieces don't seem to be quite so blue inside as
the crystal was outside, for the broken bits seem to get
lighter coloured, and now the powder is quite pale blue,
almost white. I can't understand that, because before,
the big bits looked quite dark blue. Perhaps something
has been rubbed off from the mortar?
I o CON VERSA TIONS ON CHE MIS TR Y.
M. No, porcelain is hard, and is not affected by
rubbing. But look at these broken bits of blue glass.
Here it is even darker than the copper sulphate was, and
here it is almost colourless, yet it is the same blue glass.
P. That is quite easy to explain : the glass is far thicker
in one part than in the other. Ah, now I understand;
the little pieces of copper sulphate are just as light blue
as the glass in their thin parts, and the large pieces dark
like the thick glass.
M. Right. When light penetrates a piece of the blue
substance it gets reflected again and again inside, till it
can come out somewhere, so that the further in it has to
penetrate the bluer it becomes. That is why the larger
or thicker pieces are darker than the smaller. In the
same way the main mass of the sea is dark blue or green,
and the small quantities of broken-up water seen on
the foam of the waves or in the track of a ship look quite
white. That is why, when you are talking of the colour
of a substance, you must mention at once whether you
are thinking of it in a state of powder or in big lumps.
Generally, when we give the colour of a thing in chem-
istry, we describe its colour as seen when it has been arti-
ficially prepared. A great deal still remains to be said
on the question of colour, but we have had enough for
to-day.
3. SUBSTANCES AND MIXTURES.
M. Go over what you learned yesterday.
P. Substances are known by their properties. One of
these properties is colour. This looks different, how-
ever, according as the substance is in large or small
pieces.
SUBSTANCES AND MIXTURES. II
M. Right. Do you know this stone? It is called
granite. What is its colour?
P. Grey, and reddish, and black.
M. Why do you name several colours?
P. There are several in the stone; there are grey, and
red, and black bits. You can't say that it has any one
colour.
M. Is granite a substance ?
P. Of course; because all sorts of things are made of
granite, for example, the street pavements. And a small
piece of granite is still granite.
M. Let us see. Now, just imagine granite crushed
into such small pieces that every separate piece is either
black, red, or grey. Then we put all the grey pieces in
a heap together, and the same with the red and the black.
Would you call each of the three heaps granite, or only
one, and which ?
P. Perhaps the red. No, that wouldn't do. Granite
is only granite when it is all together.
M. Quite right. Could you do the same with a piece
of sugar, and how many heaps would you have then?
P. No, it wouldn't work with sugar. Sugar always
remains the same.
M. Right again. Now, notice well, you have discov-
ered a very important difference. Substances like gran-
ite, which can be divided up into different heaps after
they have been broken up, are called mixtures. Those
where it is not possible, as with sugar, are of the same
kind through and through; we call them homogeneous.
In chemistry we only concern ourselves with homogene-
ous substances.
P. Why with these only ?
M, Because the number of the others is endless. Just
1« CONVERSATIONS ON CHEMISTRY,
think: You have two different homogeneous substances.
Then you can make innumerable mixtures according to
the proportions in which you mix them. If we had to
take note of every single mixture, we should never come
to an end.
P. But after all they are something; we can't leave
them out.
M. Very good. You are quite right. But we don't
need to know each mixture separately, and this is true
for the following reasons: When we bring together two
homogeneous substances into a mixture, all the prop-
erties of the mixture are such as can be calculated from
the properties of the two separate substances, according
to the proportion in which they are mixed. For in-
stance: A mixed colour is the result of the simultaneous
and separate action of the single colours of which it is
made; the mixing of colours in painting depends on this.
For this reason we needn't examine very closely into the
properties of mixtures.
P. Please explain that more clearly.
M. When a shopman has marked a yard of material
at a certain price, he doesn't need to write down how
much a half- or a quarter-yard costs ; and so you can easily
find out the properties of the mixtures from those of the
ingredients; and you don't need to look out and write
down those of all possible mixtures. Everything that
can be asked about a mixture can be answered by cal-
culation if you know its ingredients and their relative
amounts. Our silver money, for example, consists of
^Ya; of silver and ^/^^ of copper, and the value of a pound
of this metal is made up of ^Ygy of the value of a pound
of silver and ^/^^ of the value of a pound of copper.
P. I see that. But I can't always tell if it is really a
SUBSTANCES AND MIXTURES, 13
mixture. When I take my paint-box and mix blue and
yellow, green appears, not a mixture of blue and yellow.
M. That is only because the grains of colour are too
small for you to recognize singly when they are near each
other. If you looked at the mixture through a micro-
scope, you would sec the blue grains beside and on
top of the yellow ones. Blue and yellow glass laid over
each other make green. Therefore when the light from
a yellow grain goes through blue, or vice versa, it be-
comes green.
P. But supposing both stuffs were white, then I
couldn't recognize them together even under a micro-
scope, and I couldn't tell whether it was a mixture or not.
M. If I took a mixed spoonful of sugar and white sand,
then I certainly couldn't see that there were two things
in the mixture. But when I pour sugar into water, what
happens then?
P. It dissolves, and later the water becomes quite
clear again, and tastes sweet.
M. And what happens to sand?
P. It makes the water cloudy.
M. And doesn't make it sweet. Now, if I pour my
mixture of sugar and sand into water, the water will
become cloudy, and sweet like sugar. So I can tell
them both together.
P. Yes, it is so.
M. Why is it so? Now, I will tell you. Colours are
not the only properties which substances possess, and
by which they can be recognized and distinguished.
The behaviour with water is a special property, and this
is different with sugar and sand, even though the colours
are the same. Therefore when you want to distinguish
between a great many different substances, you must
1 4 CON VERS A TIONS ON CHE MIS TR Y.
know not only one or two, but a great many of their
properties, so as always to find out a difference even
though other properties seem the same. That is why
so many different properties of substances are examined
and described in chemistry.
Now for another question. Looking at the ingredients of
granite, we might think that we could separate them by
their colour, so that we had them in different portions.
Do you think that you could in any way separate the
mixture of sand and sugar?
P. It ought to be possible, but I don't know how.
M. Just look at the glass in which I have stirred up
the mixture with water. Now, the sand has sunk to the
bottom, and the sugar is dissolved in the water.
P. Yes, now I see; you only need to pour off the water
with the sugar, and the sand will be left behind in the
glass.
M. Will they both be completely separated then?
P. No, you can't pour out all the water. The sand
will be wet, and some sugar will still be in the water.
M. Now, g,ttend and see how it is possible to do it.
I have here a round piece of a particular sort of paper,
which is called filter-paper. It is something like blot-
ting-paper, as it sucks up water, only it is made of a
purer and firmer substance. I fold the paper in half,
and then again in half, and pull it apart so that a sort of
little trumpet is made, which is quite plain on one side,
and on the other has three layers of paper. That is
called a filter. I put my filter in a glass funnel and wet it
with water. Now I can press the filter on to the sides of
the funnel so that it quite covers it. The funnel is now
put in a stand and a glass placed under it (Fig, 2).
P. What is the good of all this?
SUBSTANCES AND MIXTURES. 15
M. To separate the sand entirely from the sugar. If I
pour the mixture of sand and sugary water into the filter,
Fig. 2.
the water will come through and the sand remain in the
filter.
P. But the sand is still wet and some sugar is still
there.
M. That we will soon wash out. I only need to pour
some pure water into the filter, and this will run through
and take the sugary water with it. Also, to rinse the last
grains of sand that remain in the glass into the filter, I
use fresh water. In case it wasn't completely through
the first time, I wait till the water has run through, and
repeat the rinsing out several times. So now we are
ready. When the filter with the sand is quite dry, then
we have completely separated it from the sugar.
1 6 CONVERSATIONS ON CHEMISTRY.
P. But how are we going to get the sugar?
M. We shall get that to-morrow. I pour the water
that has run through the filter into a flat china dish, or
a plate, and place it on the warm stove.
P. Why?
M. What does water do when you put it on a warm
stove ?
P. It dries up.
M. Yes, it evaporates, it changes into water vapour,
which disappears in the air, and nothing is left in the
dish. Does sugar do that, too? Does it become less
when it is on a warm stove?
P. No, it stays there till some one eats it up.
M. Quite right. If I put my water which contains
the sugar in a warm place, the water will evaporate, but
the sugar will stay behind, and when all the water is
evaporated, only the sugar remains in the dish. In this
way we shall at last have completely separated our mix-
ture of sugar and sand.
P. I wonder what the sugar will look like to-morrow.
At present you can't see it a bit, for the water is quite
clear, and to-morrow it ought to be there still.
4. SOLUTIONS.
P. Is the sugar there ?
M. Here is the dish. Look at it.
P. Yes, I can see a white heap that looks like sugar.
There is still some wet, though.
M. That is the rest of the water which remains with
the sugar, and only goes away slowly. A great deal of
sugar is dissolved there, and the fluid is much less mobile
SOLUTIONS.
17
than pure water, and the water takes far longer to evap-
orate.
P. But it hasn't come out in powder as we put it in.
M. No, it has appeared in the form of crystals. The
crystals in the dish are not large, neither distinct nor beau-
tiful. But I have another sort of sugar here ; * do you
know it ?
P. Yes, it is sugar candy.
M. Quite right; this kind of sugar candy is generally
made this way. You dissolve it in warm water and let
it slowly separate out or crystaUize. Only look care-
fully at the sugar candy; every piece is a crystal.
P. Yes, now I recognize everywhere the smooth, even
sides. Is ordinary sugar not made of crystals ?
^
^
//
U-^
//
Fig. 3.
M. Certainly, only the crystals are far smaller. Here
is a magnifying-glass, a lens. Just look through it at the
sugar in the sugar-basin.
P. It looks like sugar candy.
i8
CONyERSATIONS ON CHEMISTRY.
M, Loaf sugar also consists of crystals, but they are
so grown together that you cannot easily recognize them.
All this sugar is separated from solutions, and therefore
it is always crystalline; that means it is made of more
or less distinctly developed crystals.
P. Are crystals always left when you let a solution
evaporate ?
M. In most cases. But to get crystals you needn't
always let a solution evaporate; there are many other
ways, one of which I will show you immediately.
Here I have a glass with the copper sulphate we used
lately. If I put some with water and shake it, it will
dissolve, and the water will become blue (Fig. 3).
P. Why do you do that in this little glass tube ?
M. You will soon see why. A chemist uses these
little tubes for most of his experiments, as long as he is
not working with great quanti-
ties, and for that reason they are
called test-tubes. Now I light
my spirit lamp and heat the
water with the copper sulphate
(Fig. 4)-
P. Take care, the glass will
crack! How extraordinary! it
hasn't broken.
M. This sort of glass doesn't break if you handle
it properly. Now look at the contents; before there
was copper sulphate with the blue water; now it has
vanished and the solution is a darker blue. I can put
more copper sulphate in now, and it will also dissolve.
But if I add more and more, finally I can bring the solu-
tion to the boil, and the remainder stays in the same
condition. Now I add some more water to it and heat
Fig.
SOLUTIONS. 19
it up again, and it all dissolves. We will now put the
clear liquid aside.
P. But why didn't the test-tube break before? Glass
cracks when you heat it.
M. Not always. You know that you make glass by
melting it, and to do that it must be very hot; every ves-
sel or piece of glass has been made very hot, and yet has
not cracked.
P. Yes, but mother scolded me the other day because
I had poured hot wate» into a glass, and it had broken.
M. That is quite true. Here is a contradiction which
we must try to unravel. In what other ways can you
crack glass ?
P. By hitting or crushing it.
M. Yes, when you want to try to make the glass a
different shape and at the same time try to strain differ-
ent parts differently. Can heat also have an effect on
the form of glass ?
P. Yes, heat causes all bodies to expand.
M. Quite right. Then a hot glass will be rather larger
than a cold one. Have you ever seen that ?
P. No; it is so little that you can't see it.
M. All the same I will show you. I have here a fairly
long glass tube. I fasten it with one end in a stand, so
that it is horizontal, and put at the free end a measured
ruler. Now notice the line where the end is pointing. So
that you may see it better, I shall stick on a black needle
with wax. Now I bring my lamp under the tube so as
to heat it. What do you see?
P. The end first rises, and then goes slowly down
again (Fig. 5). Extraordinary!
M. Why are you so astonished?
P. I thought the needle would go forward, because
20 CONVERSATIONS ON CHEMISTRY.
as the heat makes the glass tube expand it must get
longer.
M. Instead of that it becomes crooked, and bends
upward. How, I will explain to you.
P. Wait a moment; I know it myself. The lower
part of the tube where the flame hits it has become hot-
ter than the upper part, and so it has expanded more
below than above, and has become bent.
M. Right; and afterwards the upper part got hot also.
Fig. 5.
and bent itself straight again. Then glass is slightly
bendable. But if I bend it too roughly —
P. It breaks.
M, Now you can see why a glass breaks with heat.
When you heat it unequally it bends, and when this hap-
pens too quickly, it breaks. But if the glass is equally
warmed this doesn't happen. The hot water warmed
your glass in the inside while it was quite cold outside,
and that is why it cracked.
P. But your tube was cold inside when you put it in
the flame and heated the outside of it. Why didn't it
crack too?
M. Because it is made of very thin glass. The heat
passed quickly through the whole glass. You can also
SOLUTIONS. 21
bend thin glass far more than thick before it cracks.
That is why all chemical glass apparatus needed for
heating purposes is made of thin glass, and care is taken
that it is not too quickly or unevenly heated, so that the
warmth may spread itself equally over the whole glass.
But now we will look how our copper sulphate solution
is getting on, that in the mean time has become cold.
P. There is solid copper sulphate again in the glass.
M. I will pour the liquid part in another glass, and
take out the hard part with a glass rod. To dry it, I lay
it on a piece of filter-paper, that will suck up the liquid.
Watch it carefully. What do you see?
P. There are crystals again.
M. Yes; these crystals have not come because the
solution is dried up, but because it has cooled.
P Please explain that to me.
M. If you take a certain amount of water and dissolve
copper sulphate in it, can you dissolve as much copper
sulphate as you wish?
P. No; after a time it won't dissolve any more.
M. Right. A given amount of water can dissolve only
a given amount of another substance. Such a solution is
called ''saturated." If, however, you warm such a solu-
tion, then it can dissolve more. But when you cool it
again, the solution cannot contain the extra amount it
has taken, and this separates itself in a solid form and
takes the shape of crystals.
P. That is just the same as after evaporation; for
the water went away, and there was no more there for
the substance to keep in solution.
M. Quite so. Whenever there is more substance
than a saturated solution can hold, it separates itself in
solid form. Later on we will learn another condition
22 CONyERSATlOm ON CHEMISTRY.
that must be fulfilled by this. But I haven't yet asked
you what you learned yesterday.
P. Yesterday we talked about mixtures and homo-
geneous substances. Mixtures consist of different sub-
stances.
M. And how can mixtures be recognized and sepa-
rated ?
P. By the constituents having different properties; for
example, we can pick them out if they have different
colours, or one will dissolve in water and the other remain
behind.
M. Yes, if the other doesn't also dissolve in water.
But the solutions that are produced, are they mixtures or
homogeneous substances?
P. Mixtures.
M. Why?
P. Because you can put them together out of different
substances, and again you can divide them up into their
ingredients.
M. That is right so far; but have solutions like other
mixtures the same properties as the ingredients before
they are mixed?
P. Yes, the solution of copper sulphate is blue like the
copper sulphate and a solution of sugar tastes sweet Hke
sugar.
M. Copper sulphate and sugar are soHd bodies, but their
solutions are liquid like water. If you take another solid
body like sand, and stir it up with water, it will make a
thick mixture, and not a solution.
P. Yes, there is a difference there. But perhaps the
sugar gets divided up into such small pieces that they
can neither be seen nor felt.
M, You may believe it, but you cannot prove it. For
MELTING /iND FREEZING, 23
when you look at a solution even through the strongest
microscope, you don't see any separate particles.
P. But perhaps the particles are still smaller?
M. It is useless to speak about it any longer, as we
can't decide it.
P. There is something special about solutions, then,
which can be distinguished from ordinary mixtures?
M, Yes; solutions are homogeneous mixtures.
5. MELTING AND FREEZING.
M. What did we speak about yesterday?
P. About solutions, but I didn't quite grasp it aU.
M. What is the difficulty?
P. That out of a solid substance or a liquid a real
liquid is made.
M. Just think for a minute if you can't make liquids
out of solid substances in any other way.
P. Oh yes, when ice or snow melts.
M. Does that only happen with ice or snow, or can
other soUd substances melt ?
P. Yes; on New Year's eve we melted lead.
M. Through warming or heating you can make solid
things melt, or turn them into liquid. And when the liquid
is cooled?
P. It becomes soHd again.
M. Then we can change ice into water, and water
into ice, if we warm the ice, or cool the water. At what
temperature will ice become liquid?
P. At 0°.
M. And when does water freeze to ice ?
P. Again at 0°.
2i
24 CONVERSATIONS ON CHEMISTRY.
M. Does the ice become liquid when it is warmed to 0° ?
P. It ought to.
M. You have forgotten what you learned about that
in your Physics lessons. We will just try it for our-
^ ^ selves. I have here a thermometer. This sort
I, ^^ \ is made out of a narrow glass tube, with a
bulb at the bottom containing mercury (Fig.
6). As mercury expands with heat much
quicker than glass, it rises higher in the
tube, and the higher the temperature is, the
higher it rises. A row of equidistant strokes,
with numbers, a scale, makes it possible to
read the height of the mercury, and conse-
quently the temperature. I now dip the buib
of the thermometer into the crushed ice here in
the beaker. In a short time it sets itself opposite
the stroke with the mark 0°.
P. Why does the mercury stand at the 0° ?
M. The thermometer-maker arranged that.
When he had the instrument so far ready that
only the scale remained to be put on, he put it
in melting ice, and marked the place where the
\ / mercury was. After that he placed the scale so
I that the zero came exactly on this place,
i P. Then there is no heat there.
^°* * M. No, it is a temperature that we have called
0°. It is quite an arbitrary choice, because you know
that in winter the temperature falls far below zero.
The lowest temperature that has been reached sc far
lies about 260° below 0°.
P. Why did they hit upon this choice?
M. That you will soon see. I surround the beaker with
my hands, and try to warm it. Look at the thermometer.
2i;o
MELTING AND FREEZING, 25
P. It is Still at o^.
M. Now I pour some water out of the bottle that has
stood in the room for this purpose. About what tempera-
ture is this water?
P. In a room it should always be about 17° or 18°.
The water will be about the same.
M. Look at the thermometer.
P. It is at 5°.
M. The warm water has raised the temperature then.
Now stir it carefully round.
P. Now the thermometer is getting lower; now it is
again at 0° and is remaining there. How is that? The
room is warmer, and the thermometer ought tt) rise.
M. When ice and water are together, the temperature
always remains at 0° as long as both are present. If you
try to raise the temperature by adding heat, so much ice
melts as to use up the whole added heat. If you take
heat away, so much water freezes as to replace the heat
removed.
P. Is heat made when water freezes?
M. Certainly; when water freezes to ice, exactly the
same amount of heat is formed as is used when the ice is
melted again.
P. How is it that it is exactly the same?
M. Just suppose for a moment that the two quantities
were different; suppose that on freezing, the resulting
heat was represented by the number 80, and on melting
only 60 was used. If we freeze water, and then let the
ice melt, it is exactly the same at the end as at the begin-
ning; but of the heat, 80 parts have been produced, and
only 60 used, so that 20 remain over. Now this can be
done as often as you Hke, so that you could produce any
quantity of heat from nothing. But that is not possible ,
26 CONyERS^TIONS ON CHEMISTRY.
and therefore in melting, exactly as much heat is used as
was given out on freezing.
P. Is it quite impossible to make heat out of nothing ?
Rubbing makes heat.
M. But not for nothing. To rub, you must work,
and you cannot create work out of nothing. But let us
leave this subject, for I will explain to you later what a
quantity of heat is, and how it is measured. We will go
back to our water and ice. You saw that when both were
together, the thermometer always remained at a particular
temperature, which is generally called o°. Therefore there
is quite a definite temperature when solid ice changes into
liquid water, or melts. Now, do you think that there is
always a particular temperature when a solid substance
melts ?
P. There must be something of the sort, as lead is easily
melted, and silver is difficult to melt.
M. Now we come to a general law, that every substance
melts at a particular temperature and freezes at the same
temperature. The melting-point and freezing-point of a
substance are always the same. It is that temperature
at which the solid substance and the liquid substance can
exist together, and at which heat added or removed is
used only in changing the liquid or the solid from one into
,the other. The melting-point then is as much a property
as its colour or solubility.
P. Who made this law?
M. The name law is only used figuratively. People
found that it was the case with substances, and have
consequently compared them with obedient pupils, who
always do what they are told. In science, people under-
stand by a law something that applies to many things,
and can be expressed in a general form.
MELTING ^ND FREEZING. 27
P. Are there many laws li'ie that?
M. Yes, a great many. To know such laws makes the
task of noticing and using individual facts much easier.
P. Please explain that more distinctly.
M. Let us take the law that a mixture of water and ice
has always a dofmite temperature. If a thermometer-
maker in London has made his thermometer so that a
mixture of ice and water shows a temperature of 0°,
he can be perfectly sure that wherever in the whole world
ice and water are brought together the temperature
will be 0°. Were this not the case he couldn't sell
a thermometer, and we couldn't use a bought one for our
purpose.
P. It is really nice of the law to help the thermometer-
maker so much.
M. A law of nature is not a being who either does some-
thing, or leaves it undone. People have discovered that
ice and water together have always the same temperature.
Therefore, in this case, the thermometer-maker is placed
in such a position that he can always make generally
useful thermometers. But with one point, the point of
zero, the thermometer is not finished; all the other lines
have to be marked.
P. Aren't these just ordinary millimetres, like a ruler?
M. No, that wouldn't work. For sometimes the tube is
narrow and sometimes wider, sometimes the bulb with
the mercury is large, sometimes smaller. The mercury
would then rise to different heights if the thermometers
were equally warmed, and so they wouldn't agree.
P. That is true. Then you must warm all thermom-
eters the same amount, and mark the place of the mercury,
and then put on equal numbers of marks till you come to 0°.
M. Good. To what temperature should you heat them ?
28 CONVERSATIONS ON CHEMISTRY.
P. To any.
M. That wouldn't be right. Of course all thermome-
ters would agree that had been made at the same time,
but at another place no one would know what the com-
mon temperature was where the top mark was made.
P. Then I can't think of anytl>ing better.
M. It would help us if we could only find a tempera-
ture that was as easy and certain as the ice-point.
P. Ah, now I remember; it is the boiling-point of water.
M. Yes, it is the temperature at which water boils.
That is what we shall speak about to-morrow.
6. BOILING AND EVAPORATION.
M. What did you learn yesterday?
P, I learned that melting ice always shows the same
temperature, which never alters whether much or little
water or ice is present.
M. And what about the freezing of water ?
P. That shows the same temperature. But what hap-
pens when all the water is frozen?
•M. Then we have only ice, and this we can cool as
much as we like. In the same way, when we melt ice all
the ice becomes Hquid ....
P. Then we have only water, and this we can warm
as much as we like.
M. That is nearly right, but you jumped to a too
rapid conclusion, because it doesn't hold in all cases-
We will speak about this shortly. But first let us go
over again what we spoke about. What is the condition
that gives the temperature of o°? Try to explain this as
quickly and generally as possible.
BOILING AND EVAPORATION, 29
P. Let me think a minute. Ice is at 0° when it melts,
and water when it freezes. But when ice is melted, or
water is frozen, it isn't at 0° any longer. There must be ice
in water, or water along with the ice. Oh, now I know;
when ice and water are together, then the temperature is
at 0°.
M. Right; that is the condition. Can you see exactly
why this condition must be fulfilled?
P. It seems to me it must be quite simple, only I can't
get it out.
M. It is really quite simple. What happens when you
try to warm a mixture of ice and water?
P. You explained that to me yesterday. It only melts
some ice, and that uses up the heat that has been put in.
M. And when you try to cool it ?
P. Then some of the water freezes to ice, and gives . . .
M. And gives out exactly the same amount of heat
as has been taken away. You see the thing is like
the height of water in a pond that always remains at the
same level. If you take water away, more flows in from
the spring; if you pour water in, it runs over the dam,
and the height of the water is still the same.
P. I understood that, but I haven't got it quite clear
yet. Does a lot of water with a little ice give the same
temperature as a lot of ice with a little water?
M. You have not been attending. We learned all this
yesterday as a law of nature; that is to say, as a thing
that is always the same.
P. Oh, now I remember; now I see it all. Why, it is
ridiculously easy; I thought it would be far more difficult.
M. That will often happen. When you have got a
thing quite clear, it always seems very easy. But the
YpXing it clear is not always so simple and easy. But
30
CONVERSATIOhlS ON CHEMISTRY,
now let us go back to my first remark. Can you really
heat water without ice as much as you want? What
happens when I put water in a pot over the fire?
P. First it will get hot, and then it will begin to boil.
M. Right. We will make the experiment. I have
here a flask made of thin
glass which I can put over
the flame without its cracking.
In it is some water, and I
shall put it over a tripod
which stands above my lamp
(Fig. 7)-
P. Why is that wire gauze
on the tripod?
M. For one thing, so that
I can put large and small
vessels on it. Again, the
metal spreads the heat of
the flame, and prevents the
glass from breaking so
easily if it is a little thicker.
Now I put my thermometer
in the water.
P. Do you see? The water is getting warmer.
M. Wait a bit.
P. Now the water is boiling, and the mercury has
risen quite high; it is already at ioo°. Now it will soon
fill the whole thermometer. What will happen when the
mercury has no more room to expand?
M. The thermometer will break, for it exerts very
strong pressure.
P. Then take the lamp away at once.
M. Look at the thermometer first.
Fig. 7.
BOILING AND EVAPORATION, 31
P. It is Still at 100°.
M. And will stay there as long as you like. I am
making the flame bigger. What do you see?
P. The water is boiling harder.
M. And the thermometer?
P. That is still at 100°. Oh, now I am beginning to
notice something. It seems to be exactly the same here
as with the melting.
M. Quite right. Now try to trace the resemblance.
Then the temperature was unchangeable when two
things, ice and water, were together. What is it here?
P. There is water here too, but what is the second?
Wait a bit, I've got it now; it is steam. Is that right?
M. Yes. When I supply heat by means of a flame, it
doesn't heat the water any more, but changes it . . .
P. Into steam!
M. Now we must reverse this relation. We had the
same temperature before, whether we started with water
or with ice; now . . .
P. Now we must get the same temperature whether
we start with water or steam. We have got the one when
we started with water, but how do we get the other?
We must take a vessel with steam, and try to cool it.
That isn't easy to do; we must have a boiler for it.
M. We can do it in a much easier way. Look, I take
the thermometer out, and let the water boil quickly for a
minute or two. The thermometer has now cooled a little,
and it has fallen to under 50°. Now I put it again into the
flask; not into the boiling water, however, but hold it
above in the upper part of the flask. What do you see ?
P. Water is dropping from the thermometer. How
did it get there? I know; the steam in the upper part
of the flask has condensed on the cold thermometer.
32 CONVERSATIONS ON CHEMISTRY,
M. Right. Read the temperature.
P. It is at 1 00° again.
M. Now we have made the experiment for which you
wanted a boiler. The upper part of the flask contains
steam, which rushes upwards and makes clouds outside.
By the cold of the thermometer a part of the steam is
made into liquid water, and also in the upper part of the
flask too. You have thus steam and water together.
Steam condenses to water on the thermometer till the
lost heat is supplied again, and the temperature has risen
to 100°.
P. Is there really steam in the upper part of the flask?
It is quite clear.
M. Steam is as transparent as air.
P. Is that so? I thought steam was always misty and
untransparent. When a steam-engine blows out steam,
you see it like a thick white cloud, and in the same way
the clouds in the sky are steam.
M. No, what you see isn't steam, but liquid water in
very small drops that have been made out of steam by cool-
ing. If you could look into the boiler of a steam-engine,
you would see that the inside is quite clear, just as if it
was full of air. Also in the clearest air there is always a
large amount of steam; and mist and clouds are made up
by the cooling and building up of liquid water in the shape
of tiny drops. So you see these things behave in very
much the same way as water and ice. Water and steam
only exist together at a definite temperature, and when they
are present together that must be the temperature.
P. How does it happen that it is exactly 100° ?
M. In every thermometer the 100° is marked just where
the mercury rises to in boiling water.
P. How can they do that?
BOILING AND EyAPORATtON.
33
100-
-212
50-
M. Don't you remember how we left the thermometer-
maker? He had only been able to put one mark on his
tube, and had written o° there when the mercury was in
melting ice. Now, he must have another distinct tem-
perature to have another mark, in order that he may
divide up his instrument. This second temperature is
that of boiHng water, and people came to the conclusion
that the portion between the two marks
was to be divided into a hundred parts.
As the lowest mark is called o°, the top
one must be ioo°.
P. Now I understand. But how can
higher or lower temperatures be measured ?
M. As many equal divisions are marked
below the zero-point and above the loo-
point as there is room for. According as
the thermometer is required for high or
low temperatures, more or less mercury
is put in, so that there is enough space
over on the required part (Fig. 8).
P. But our window-thermometer is not
divided up to ioo°. It stops at 50°.
How could they make the right division
in that case?
M. First a thermometer is made with
great care from 0° to 100° and correctly
divided. That is called a normal ther-
mometer. Then the short thermometer
is brought into the same medium as this
— for instance, both are dipped into a rather large
quantity of water. Since obviously both thermometers
must now register the same temperature, we have merely
to mark on the small one the number at which the mer-
cury stands in the large one.
— 100
Fig. 8.
34 CON VERS A TIONS ON CHE MIS TR Y.
P. Is that how it's done? Now, I don't think I've
anything more to ask. Yes, I have though ; on the left
of our window- thermometer is a C and on the right an
F., and it is divided differently on both sides.
M. F. means Fahrenheit. Fahrenheit was a German
who made the first comparable thermometer; he lived
in the eighteenth century. He wanted to divide his
thermometer from the lowest temperature that there
was; so he put it in a mixture of snow and sal ammoniac,
and marked the point to which the mercury sank as o°.
The piece between this point and freezing-point of water
he divided into 32 parts, and found that 180 of these
parts were contained in the space between freezing-point
and boiling-point. People used the division of Fahren-
heit for this reason alons, because the freezing-point was
32° and the boiling-point 32° -f 180°, or 212°.
P. Why don't they still use Fahrenheit's plan?
M. Because the mixture of sal ammoniac and snow
is very difficult to bring to a definite temperature,
while the freezing- and boiling-points of water are much
surer.
P. Does every one use these thermometers?
M. The English and Americans do. They use them
only in ordinary life, however, mostly for open-air ther-
mometers. In all scientific work they use the centigrade
thermometer. Give me the equation between Celsius
and Fahrenheit, and use the letter / for Fahrenheit, and
c for Celsius.
P. /:c=i8o°:ioo°, or 5/ = 9C.
Mo That is not right.
P. Why not?
M. The freezing-point of Celsius is zero. If you say
c = o°, then your equation comes out to / = o°. But the
BOILING AND EVAPORATION. 35
freezing-point of Fahrenheit is not o°, but 32°. What
must you do so as to make / = 32° when c = o°?
P. I must put the 32 on the other side.
M. Well, let me hear the equation.
M. Put the c = o in here. Now what happens?
P. 5/ = 32. No, that is not right; the / must stand
alone on the left. How can I do that? Now I know:
First, I must write ]=^/ f,c and then add 32 to the right;
so /=V5C+32. Now, I put c = o° and that comes right;
M. Yes, now the equation is right.
P. I have read about a thermometer called Rdaumur
that was quite different.
M. Yes. For rather more than a hundred years the
thermometer of a Frenchman, Reaumur, has been used.
In his, the space *between freezing- and boiling-point was
divided not into 100, but 80 parts. On the other hand,
the Swede Celsius introduced the division of 100. In
Germany the Reaumur thermometer came into use, while
in France the centigrade one was used. Presently people
grew accustomed to register all temperatures by the
centigrade thermometer; in science no other is now used.
What is the relation between the degrees of Reaumur and
Celsius ?
P. 100° C. are 80° R.
M. Simplify the proportion.
P. 10° C. are 8° R., or 5° C. are 4° R.
M. You can write this as an equation too. Take c for
centigrade and r for Reaumur degrees — that makes
c:r::5:4, so c=^/^r^ or r^*/^c. The first equation you
36 CONVERSATIONS ON CHEMISTRY.
use when you wish to change Reaumur into centigrade,
and vice versa.
P. Has the mixture of ice and the other thing really —
M, Sal ammoniac ?
P. And sal ammoniac the lowest temperature that there
is?
M. Far from itl It is sometimes colder here in winter.
Think how many degrees of Celsius there are to the
zero of Fahrenheit.
P. I must put / = o; then 0 = ^/^0+32; that makes
M. Yes, not quite 18° under 0°; but in America it is
often 20° to 25° below zero.
P. What is the greatest cold that there is?
M. Up to the present 259° below 0° has been attained.
P. What do you mean?^ Will they get further?
M. Not much. Probably — 273°C. is the lowest tem-
perature there is.
P. Why do you think that ?
M. I can't explain to-day, but you will soon discover
and believe it as well.
P. Oh, I wish I knew!
7. MEASURING.
M. What did you learn yesterday?
P. How thermometers are made.
M. Yes. As a thermometer is a sort of measuring
instrument we will speak a little about measurement.
What can be measured?
P. All sorts of things: Lengths, weights, surfaces. I
think almost everything can be measured.
MEASURINC. 37
M. Not all, but a great many things. What is used
for measuring?
P. A measure.
M. What is that?
P. There are different sorts; it depends on what you
want to measure.
M Give me an example.
P. Well, the length of the table can be measured in
feet and inches.
M. Although feet and inches are used in England and
America, all scientific people measure in what is called
the metric system.
P. What is that?
M. We are going to learn it. Here is a centimetre rule.
Measure the length of the table.
P. The scale is 50 centimetres long; I see that on the
last figure. I lay the measure so that its end is against
the end of the table, and notice to where it reaches.
Then I put the measure at that mark, and again make a
scratch where it ends. My measure comes beyond the
table, now that I have put it at the second mark, and I
look at which number the table ends. It is at 22. So
the table is 50+ 50-f 22^=122 cm.
M. Quite right. You went on adding centimetres
together till you had got the same amount as the length
of the table. The measure only helped you to count
the centimetres.
P. Yes, so it did.
M. And how do you set about measuring weights?
P. I put the thing in one pan of a balance, and add
weights to the other, till they are both the same
weight.
M, And how can you notice, or tell the weight ?
38 COhiyERSATIONS ON CHEMISTRY.
P, The number of ounces each one weighs is marked
on the weight; I add the figures all up afterwards.
M. Let us use grams. You see it is the same as before;
you add grams together till their weight is the same as
that of the object. The weights only help you to count
the grams.
P. So they do. I never noticed that both were so like.
M. You will soon see that all real measuring is based
upon the same principle. But now for another question:
Why didn't you measure the length with grams and the
weight with centimetres?
P. It wouldn't work.
M. Why not?
P. However many centimetres I put together they
would never make a weight.
M. Quite right. Put this in a simple form.
P. Length can be only measured by length, and
weight by weight.
M. It could be said still more simply. Every quantity
can be measured by a like quantity.
P. Yes, I understand that.
M. You measured length in centimetres, hre centi-
metres the only measure of length?
P. No, there are millimetres, kilometres, inches, miles,
fathoms, and a great many others.
M. How far do these differ ?
P. A centimetre has a different length from an inch,
and so on.
M. Yes; these definite lengths, such as a centimetre,
inch, and mile, are called units of length. In every state-
ment of measurement we get the kind of unit which has
been used, and the number- of units which are contained
in the thing measured.
MEASURING.
39
Fig. 9.
P. Then why are there so many sorts of units for the
same sort of quantity; for example, length?
M. That is because the choice of the units is arbi-
trary. At first different groups of people who required
a unit of length chose one without
troubling themselves about what
other people were using. Finally
these differences grew so un-
bearable that in France, at the end
of the eighteenth century, the State
determined to abolish the old meas-
urement and to use a new one in
its place. It was determined to
protect the standard against acci-
dental destruction, and so it was
decided to use the world itself as a measure. The
length of a quadrant of the meridian, that is, the length
from ^ to A^ (Fig. 9), was divided into ten million
parts, and these parts were called metres, and were
to serve as a common unit of length. A centimetre is
a hundredth part of this length, so that it is a thousand-
millionth of the earth's quadrant.
P. But how can the earth's quadrant be divided,
when no one has been to the north pole ?
M. Only a part of it is measured, the relation of which
to the whole is determined by the angle which lines at
right angles to two tangents form with each other. But
it turned out that this measurement was far less accurate
than the comparison of two metre scales. Accordingly
the metre was taken to be the length of a standard kept
in Paris, made of the most indestructible material which
could be found — an alloy of the noble metals platinum
and iridium.
40 CONyBRSATlONS ON CHEMISTRY.
P. But supposing this scale was lost or got destroyed?
M. Care is taken about that. Twenty similar scales
have been made, all carefully compared with each other,
and there is one at Berlin, London, New York, St. Peters-
burg, Rome, and many other places, so that any one of
them might be lost without the loss of the standard.
Then, again, many other scales made of different materials
have been compared with them, so that the permanence
of the unit is about as certain as that of the human race.
P. But the metre is quite an arbitrary measure. Why
hasn't one been chosen which is free from man's control?
M. Because there is practically none.
P. But with angles it is different. I have learned in
my geometry class that a right angle is a natural measure
which cannot be altered. Why can't that be done with
lengths ?
M. Tell me any natural measure of length.
P. . No, I am afraid I can't. But why is there a
difference ?
M. It depends on the fact that an angle cannot be
made infinitely great. If you rotate a straight line round
a point in another straight line, the angle between both
increases at first, but it can't become larger than four
right angles, for that angle is equal to the angle o, and
afterwards the same angles come as before. The largest
possible angle has consequently a finite value,^ and that
value is the natural unit. But with length it is different,
for you cannot think any length so great that it could
not be made greater.
P. So nothing which can be made infinitely great can
have a natural unit?
M. Quite right. You will soon become convinced
that for all such magnitude arbitrary units must be
MEASURING. - 41
chosen. The best proof is that no one has been able to
find a natural one. Now we will go back to the
metre. It is not convenient to measure all magnitudes of
the same kind by means of the same unit. You can
measure the length of the table in centimetres; but if
you measure the height of a hill or the length of a river
in centimetres, your numbers will be far too large, and
for such great lengths larger units are employed.
P. Yes, I know; metres and kilometres.
M, Right. People have used such different units for
a long time, but they did not stand to one another in a
sufficiently simple relationship. At the same time as
the metre was introduced, it was decided only to admit
such measures of the same kind, as stand to each
other in the proportion 1:10:100:1000, and so on; that
is, in powers of 10.
P. Why was that done ?
M. Because in reducing from one measure to another
there is hardly any work to be done ; you need merely add
zeros, or alter the position of the decimal point. Thus you
have:
I kilometre (km. for short) = 1000 metres (m. for short).
I m. = io decimetres (dcm.) = ioo centimetres (cm.) =
1000 millimetres (mm.).
P. What is the meaning of kilo?
M. Kilo is the Greek word for a thousand. It was
agreed at the same time that the multiple of each unit
should be expressed by Greek prefixes (deca-, hecto-,
and kilo-) ; while the fractions are expressed with Latin
prefixes (deci-, centi-, milli-).
P. Now I understand the meaning of the words kilo-
gram and milligram.
M. You see the unit of mass is Jled the gram. It is
42 CONFERS A TIONS ON CHE MIS TR Y.
derived from the centimetre; it is the mass of a cube of
water at 4° C. The multiples deca-, hecto-, and kilo-
gram are derived from it, but only the last (kgrm.) is in use.
A kilogram is equal to two pounds. The deci- and centi-
gram are also not often used; but the milligram (mgrm.) =
0.00 1 gram is much used in scientific work.
P. You said that the gram is the unit of mass. I thought
it was the unit of weight, for people weigh with grams
and kilograms.
M. Mass and weight are related to each other. Mass
is the property of bodies which keeps them in motion
when they are once moving; and mass is measured by
the work which must be expended in order to produce
equal velocities. Now weight or the force with which
bodies are drawn to the earth are at any given place
exactly proportional to the mass, so that when two weights
are equal the masses are also equal. And for that reason
masses can be measured by help of weights.
P. Why do we require to know masses? Surely we
buy bread and iron and gold by weight.
M. Yes; hy weight, but not on account of weight. In
science weight is derived from mass, and not mass from
weight, because the mass of any body is unchangeable
although its weight may be altered.
P. But if I keep a thing carefully shut up, so that
nothing is lost, surely its weight remains unchanged?
M. I don't mean it in that sense. Of course, if you
take anything away from a body, its mass will be de-
creased in the same proportion as its weight. No; a
body has a smaller weight on a high mountain than in a
valley. And weight is less at the equator than at the
pole.
P. I remember learning that in my Geography lesson;
MEASURING. 43
it had to do with the attraction of the earth. Because
the earth is flattened at the poles, a body there is nearer
the centre of the earth than at the equator.
M. Quite right; but you must add that the attraction
decreases with the distance from the centre of the earth;
moreover, near the equator the centrifugal force increases,
so that a body near the equator is more swung off from
the earth than if it is near one of the poles, and it conse-
quently weighs less.
P. If I weigh a kilogram of sand here, and carry it up
a high mountain and weigh it again, would it really weigh
less ?
M, Not if you were to weigh it on an ordinary balance
with arms; it would counterpoise exactly as much weight
there as here.
P. But you said —
M. Don't you see that your weights become lighter in
the same proportion as your sand?
P. How can that be? Oh, I see; I hadn't thought of it.
But I can't understand how it can be proved that the weight
has become less.
M. By determining the weight, not by help of counter-
poises, but by another method. A spring balance, in
which weight is measured by stretching a spring, would
show that your sand weighed less on the top of a hill
\an in a valley. The most exact measurement is made
ith a pendulum, for it swings more quickly the greater
ihe attraction.
P. Why?
M. You will learn it in your Physics lesson. We must
go back to our old subject. I told you that things are
bought by weight, not because of weight. Why do people
buy bread?
44 COr^VERSATIONS ON CHEMISTRY.
P. To eat it.
M. Do you eat it in order to grow heavier?
P. Ha! hal ha! No, because I Hke it and because
it makes me strong.
M\ The last is the important reason. And coals are
bought, not because they are heavy, but because they
make you warm.
P. But I can't understand the use of weight.
M. Which would you rather have, a small piece of cake
or a large one?
P. Of course, a large one.
M. Why?
P. Because there is more of it. A little one wouldn't
satisfy my hunger.
M. And which weighs more ?
P. The larger one, of course.
M. Now you see the use of weight. The properties
and uses which make us buy things increase or decrease
with the mass or the weight. The power which bread
has of keeping you alive increases proportionally to its
weight, and the greater the weight of the coal you buy
the more heat you can get from it, and just as with these
marketable properties, so a great many scientific prop-
erties are dependent on the mass and on the weight
The balance is therefore a very important piece of chemical
apparatus, not so much because we want to know the
weight of things, for often we do not care to know it, but
because of the other properties which are connected with
weight.
P. So weight is like the paper of a book, which is
worth very little in itself, but becomes valuable for what
is printed on it.
M. That is a good comparison even though it doesn't
MEASURING. 45
quite fit. Let us take a better example. As you know,
liquids are bought and sold both by measure and weight.
Wine and beer are sold only by measure, that is, by the
space which they occupy; paraffin-oil is sold both by
weight and by measure; sulphuric acid is sold only by
weight.
P. Why?
M. Convenience and custom are the reasons. Measur-
ing is much quicker than weighing, and a measure is
much more easily made than a balance; and so this plan is
preferred. But sulphuric acid is a somewhat dangerous
liquid, and people don't like to pour it; therefore they
prefer to weigh it. But for the purpose of determining
quantity by measurement, for any one substance volume
and weight bear a constant proportion to each other.
Hence the actions and uses of liquids are proportional
to their volumes, just as they are to their weights. The
purchaser of paraffin-oil is not interested in the volume
it occupies or in its weight; he buys it because of the
amount of light or heat which he can get from it. But
these amounts are proportional to the volume, and so
the volume becomes a measure for the amount of light
which the paraffin-oil will produce. Now tell me what
you know about measures of volume.
P. The unit is called a litre.
M. That is only half right. The real unit of volume is
derived from the unit of length, and is a cube, the side of
which is one metre long — a cubic metre. But this measure
is far too large for most purposes, and therefore one has
been chosen nearer in volume to the old pints and gallons.
It is a cube, the side of which is. Yio of a metre; its
capacity therefore is Yiooo of ^ cubic metre. It is called a
cubic decimetre, or a litre (1.).
46 CONyERS/l TIONS ON CHE MIS TR Y.
P. You have surely made a mistake in saying that a
cubic decimetre is a thousandth of a cubic metre. A deci-
metre is only a tenth of a metre.
M. Think a minute!
P. What a stupid I was! The volume of a body is pro-
portional to the cube of its side, and 10X10X10= 1000.
M. Yes, that is right. In science we use as a measure
one-thousandth of a litre. How large is that cube?
P. I won't make another mistake. The side is ten
times less, y^o dcm. is Yioo i^i. It is a centimetre.
M. The measure of volume is called cubic centimetre
(ccm.). Now write me down a table of measures of
volumes.
P. I cbm. = 1000 ]., and i 1. = 1000 ccm.
M. Quite right. Now we have had enough for to-day,
although there is a great deal more to say about measure-
ment.
8. DENSITY.
M, Yesterday you learned how to measure and to
weigh; to-day we will talk a little more about measure-
ment. Which is the lighter, a pound of lead or a pound
of feathers?
P. You can't catch me with that old joke. Of course
they are the same weight.
M. But which is the lighter, lead or feathers?
P. Hm! Well, feathers are really lighter.
M. That is a contradiction. It depends upon the fact
that the words light and heavy are used with a double
meaning. When you say lead is heavier than feathers,
you mean that a handful of lead has a greater weight
DENSITY. 47
than a handful of feathers; if equal volumes of feathers
and of lead are compared, the lead weighs more. If we
say wood is lighter than iron, we attach the same meaning
to the word lighter, although you could easily choose a
given piece of wood heavier than a given piece of iron.
P. I understand that.
M. But in science it doesn't do to use such indefinite
expressions. The property which is greater with iron
and lead than with wood and feathers is called density^
and we say iron is denser than wood and lead denser than
feathers. How is density determined?
P. By weight and by volume.
M. Yes. And as the density is greater the greater the
weight in a given volume, and smaller, the greater the
volume of a given weight, the density is made propor-
tional to the weight and inversely proportional to the
volume; so that if w is the weight and v the volume, the
density d is expressed by the formula
d=~,
V
P. What is the use of this formula ?
M. To measure the density. Let us take an example:
What is the density of water?
P. It depends on what weight and what volume you
take.
M. No, it doesn't depend upon that. We choose once
for all the gram as unit of weight and the cubic centimetre
as unit of volume. Now, if we take an arbitrary quantity
of water, say a litre, what is its weight ?
P. One litre of water weighs looo grams.
M. And what is its volume in cubic centimetres?
48 CONVERSATIONS ON CHEMISTRY.
P. looo c.c. make a litre.
M. So we have w= looo and v = looo; how large is </?
P. J= 1000/1000=1; the density is i.
M. Now make the same calculation for 20 c.c. of
water.
P. ^^=20/20=1. It is I again. Oh, I see; because
the volume and the weight always become larger and
smaller to the same degree, the fraction must always have
the same value whatever quantity of water is taken.
M. Now you understand it. Here I have a little lead
cube; what is its density?
P. I must first find its weight. Let me weigh it myself.
It weighs 38.84 grams. And now I must find its volume.
But how can I do that?
M. As it is a cube you have only to determine the
length of one side. Here is a rule.
P, The side is 15 mm. long, and so the volume is
i5' = 3375-
M. Equals 3375 what?
P. 3375 c.mm. Oh, I should give the volume in
cubic centimetres. I'll be right this time. The volume
is 3.375 cc. %
M. Quite right. Now calculate the density.
P. 38.84/3.375 = 11.51-
M. So the cube has the density 11.51. I can go further
and say that lead has the density 11.51, for if I had
taken any other cube of lead, or indeed any other piece of
lead, I should have found the same number. Tell me
why.
P. I can see that you would have got about the same
number, but I am not sure that you would have got exactly
the same number.
M' You have forgotten what I told you before (page 2)
DENSITY. 49
about properties. Density is a property; for all samples
of the same substance it will have the same value. Now
ordinary lead is really a very pure substance, and con-
tains hardly anything mixed with it, and so the properties
of different samples have the same value.
P. But all bodies expand with heat ; so that the volume
of the lead cube will be larger when it is warm than when
it is cold.
M. Quite right. Is weight changed by heat ?
P. Not so far as I know.
M. Weight is quite independent of temperature. So
it follows that the density of lead becomes smaller as the
temperature rises, because while the nunierator remains
the same, the denominator increases.
P. Then density isn't quite a definite property.
M. Yes, it is, for at a definite temperature it has a
definite value. The same holds for every other sub-
stance. Water, too, changes its volume with temperature;
and therefore 4° has been chosen as the temperature at
which the weight of i c.c. is called i gram.
P. Why was that temperature chosen?
M. Because water has its greatest density or its smallest
volume at 4°. What are you thinking about?
P. I am thinking how it would be possible to determine
the density if the thing wasn't a cube.
M. That is a very sensible question, for very few sub-
stances can be made into that shape. Look here, I'll
show you how it can be done. Here is a glass tube
which is divided into tenths of cubic centimetres by
little lines. I pour water into it and read where the
level stands; I find 5.33 c.c.
P. You have read off hundredths, and there are only
tenths marked upon the tube.
50 CONVERSATIONS ON CHEMISTRY.
M. Every one who makes measurements must learn to
do that. As a rule, the level of the water does not lie
neatly on a line, but between two. I divide the distance
between two lines into tenths with my eye, and so I get my
hundredths.
P. I couldn't do that.
M. It isn't difficult to learn, and you must try it after-
wards. But now we will go on. I have here a glass
with shot. They are made of lead; weigh it.
P. It weighs 43.58 grams.
M, Now I shake some of the shot into the tube.
Weigh the glass again.
P. It weighs 28.42 grams.
M, What is the weight of the shot that I have shaken
into the tube?
P. 43.58-28.42 = 15.16 grams.
M, And now I read the level of the water in the tube.
It stands at 6.66. That is 1.33 c.c more. What con-
clusion can I draw?
P. Oh, now I see. The volume of the water has risen
so as to tell the volume of the shot. The volume of 15.16
grams is 1.33 c.c, and so its density is 11.40. It is almost
exactly the same number that we calculated before.
But it is not exactly right.
M. Because you didn't measure with sufficient accu-
racy. You gave the side of the cube as 15 mm.; measure
again.
P. Yes, it is a little smaller.
M. And measure the other sides of the cube.
P. They are not quite equal.
M. You see, then, that your former measurement con-
tained errors, and therefore the result cannot be quite
accurate. To measure exactly is a very difficult thing;
DENSITY. 51
and therefore we must rest contented at present with
what we have found; the right number is 11.4. I will
let you use the balance and the measuring-glass, and you
can determine for yourself the density of various sub-
stances. But take care that you always remove the
bubbles of air, or you will measure them along with
the volume of the body, which will appear too great,
and you will get too small densities.
P. Yes, I will draw up a table. What shall I meas-
ure?
M. You had better find the densities of your minerals.
But now to another question: Have liquids also definite
densities ?
P, I think so. Yes, water has the density i.
M. Right. Now think; how can you determine the
density of a liquid?
P. By determining its weight and its volume. Wait,
I know. I shall pour it into the measuring-glass and
read out its volume.
if. And how will you find its weight?
P. Exactly as you did with the shot. I shall first
weigh the flask which contains the liquid, then pour it
into the measuring-glass, and then I shall weigh the flask
again.
M. It can be done in that way, but it is possible to do
it in a much simpler manner. Weigh the measuring-
glass once for all, then pour in liquid and weigh again,
and you need only subtract the weight of the measuring-
glass.
P. That gives me one weighing less.
M. You can lessen your work still further by not
measuring out an arbitrary quantity of liquid, but a
definite volume. This is not easy with solid bodies,
52 CONVERS/iTIONS ON CHEMISTRY.
but is quite easy with liquids, because they fill a
given volume completely. For example, if you
pour exactly i c.c. into your measuring-glass,
and determine its weight, what will your equa-
tion be?
P. Then d^g/i. That i?> d= g\ the weight
is the same as the density.
M. Do you see you don't require to divide.
It is often said that the density is the weight of
unit volume. This expression is not wrong,
but doesn't cover enough, and so I didn't tell
you it before.
P. I have just tried to pour i c.c. of water
into the measuring-glass, but it is very difficult
to get exactly the right amount. I have found
either too much or too little.
M. Pour in a little too much, and then remove
the excess with a small strip of blotting-paper.
It sucks up such small quantities that it is quite
easy to obtain the right volume.
FiG."io. P. Yes, that works.
M, It is still easier with this apparatus (Fig. lo), which
is called a pipette (this is a French word and means little
pipe). I suck the upper end while I hold the lower in
the liquid, until the level rises above a mark on the stem;
then I close the end quickly with my forefinger, and while
the point touches the side of the vessel, I can easily
let so much liquid run out that it stands exactly at the
mark.
P. But I must put the liquid in another vessel to weigh
it.
M. No. You can lay the pipette itself upon the scale.
If you have determined its weight, when empty once for all,
DENSITY. 53
you need only subtract that from the total weight and you
have the weight of a cubic centimetre, or the density. It
is still simpler to make a counterpoise of wire of the same
weight as the pipette. Such a counterpoise is called a
tare. Then the remainder of the weights on the pan
will give you the density.
P. I'll certainly do that.
M. In that manner you can determine the densities
of various liquids, such as spirits and salt water. You
will find the first lighter, the second heavier, than
water.
P. Then I can make a table of densities of liquids
as well.
M. Now you know how to determine densities of solids
and liquids, what about gases?
P. Can't their densities be determined in the same
way by measuring their weight and their volume?
M. Of course they can, but it is not so easy. In the
first place the weight of a large volume of air is very
small; i litre of air weighs only a little more than i
gram, as you have seen already. Then the volume of
gases is very easily changed if the temperature or the
pressure alters. And so very different densities are got
for the same gas if it is measured at different temperatures
or pressures.
P. But that happens too with solids and liquids.
M. The changes are much smaller with them, so that
they need only be taken into account if great accuracy
is required.
P. Then how is the density of a gas determined ?
M. That is a rather difficult thing, which I shall explain
to you later. To-day I will merely say that people have
determined upon a standard temperature and a standard
54 CON VERSA TJONS ON CHE MIS TR Y.
pressure at which to measure the volumes of gases, and
so uniform results are obtained.
P. I should never have thought that measuring was
such a difficult matter.
9. FORMS.
M. I am not going over to-day what you learned yes-
terday, because it was really just a rep^titioi of what you
had learned before. We will go back to what we spoke
about in the lesson before last. You learned two very
different properties of water. What law is at the bottom
of the melting of ice and boiling of water?
P. That both happen at a definite temperature.
M. Yes, but not only water; every substance has these
properties.
P. Really all?
M. All substances that are really pure substances.
Mixtures and solutions have changeable melting- and
boiling-points.
P. How changeable?
M. If a solution is brought to boiling point, we notice,
as the boiling proceeds, that the temperature doesn't re-
main unchanged, as with pure substances, but gradually
rises, in proportion to the amount of steam that goes
away. In the same way, when a mixture fuses or melts,
it begins to liquefy at a definite temperature; this does
not remain stationary, however, but rises higher as more
heat is added and more of the mixture becomes liquid.
P. May I see that ?
M. Later. At present we will stick to pure sub-
stances. You have seen that liquid water can be changed
FORMS, 55
into solid ice and into gaseous steam. Do you know
what these two conditions are called?
P. Yes; states of aggregation.
M. Quite right; that is the usual name. What does
it mean?
P. Aggregate means assembled, but I don't know what
that has to do with liquid or steam.
M. The name is given because it is taken for granted
that all bodies are made up of tiny particles which are able
to lie on each other, or arrange themselves in various
ways. They are called atoms. According as these
atoms are near or far from each other, they make solid,
liquid, or gaseous bodies.
P. Can you see these atoms with a glass?
M. No, not even with the strongest microscope.
People take for granted, because of that, that they are
•smaller than the smallest thing that can be seen through
a microscope.
P. But are they really there ?
M. It is true I cannot guarantee them. There is no
proof of their existence.
P. Then how can you say that it depends upon them
whether a body is solid or liquid ?
M. Real things behave in many respects as if they
were collections of atoms, if atoms exist. If it be assumed
that bodies consist of atoms, it may be deduced that they
must behave as they really do.
P. That is very awkward. Why do people not simply
say: They behave this way or that way, and be done
with it ?
M, Because, starting with the assumption of atoms,
there can be deduced several conclusions which agree
with fact. Such an assumption is called a hypothesis.
S6 CONVERSATIONS ON CHEMISTRY.
P. But I can't see what is gained if there is no proof
that the hypothesis is true.
M. The hypothesis serves to make the real relation-
ship more easily noticeable. If you have to keep in mind
three names, Alfred, Arthur, and Anthony, it will be
easier for you to remember them if you notice that they
all begin with A. Moreover, a hypothesis serves as a
stimulus to research. People imagine how a number of
atoms would behave under given circumstances, and
find out whether the actual bodies behave in that way.
P. Do they always agree?
M. No, I am sorry to say, not always.
P. But after such a conclusion has been drawn, people
ought to see whether it is right or not.
M. Certainly; but this gives an opportunity of putting
definite questions to nature and of making suitable
experiments or observations. And so our knowledge
increases, and that is always an advantage.
P. But if they don't agree?
M. Then there is nothing for it but to wait and hope
that the contradiction may be explained.
P. But that is a very uncertain way of doing things.
M. So it is; yet the use of hypotheses for learning
and investigation is so great that people will always
make use of them.
P. Couldn't they do without them?
M. Of course they could; but people are so much in
the habit of using hypotheses, like the atomic hypothesis,
that they find great inconvenience when they try to realize
things without their help. And therefore they will not
give them up.
P. Then please explain to me how solid, liquid, and
gaseous bodies are built up of atoms.
FORMS. 57
M. Ah, you put me in a difficult position if you wish
me to show you the use of the atomic hypothesis, for
up to the present it has not been entirely satisfactory.
However, we needn't delay over that at present; I only
mentioned the subject in order to explain the derivation
of the name "state of aggregation." In talking over these
things with you I prefer to consider these relations without
its help; and for that reason I will not use the term, but
rather speak of forms.
P. What does the name mean?
M. It points to the chief differences of these states.
How does a solid body behave in relation to its
form?
P. I don't know anything particular to say about that;
it can be broken, cut, or bent.
M. But if it is left alone ?
P. Then it keeps its form.
M. Right. Have you ever thought how important
that is?
P. I don't see anything very important about it.
Sometimes it is a great nuisance; for example, if I want
to break sugar.
M. Think for a minute. If the stones and rafters of
this house were to change their shape, it might fall to
pieces at any moment; none of our tools would be usable;
you couldn't cut .with a knife if the blade didn't keep its
shape; your morning milk wouldn't stay in its can if the
shape of the can kept changing continually.
P. Yes, now I see, but I can't think it out to the end.
The whole world would go to bits.
M. Now I see you are beginning to grasp it. Have all
bodies the property of keeping their shape ? For instance,
how does water behave in this respect?
58 CONVERSATIONS ON CHEMISTRY.
P. Water does not keep its shape; you may pour it into
any sort of vessel you like.
M. Is water the only thing that has this property ?
P. No, all Hquid bodies are like it. Yes, now I see the
great difference. But how is it that solid bodies keep
their shape?
M. That is a senseless question. How do you know
when a body is solid?
P. I catch hold of it.
M. And you are convinced that it keeps its shape.
The word solid is merely the name for the common
properties of many bodies of keeping their shape.
P. But that must have a cause.
M. I don't understand you.
P. Why is this silver coin not liquid ?
M. Well, when you heat it, it melts and becomes
liquid. Here is a piece of thin silver gauze; if I hold it
in the flame it will liquefy, and a drop will form on the
end. See, the drop has fallen.
P. So it has.
M. The question whether a body is solid or liquid
depends solely upon its temperature. Below its melting-
point it is a solid, and above its melting-point it is a liquid.
P. Is that the case with all bodies ?
M. Yes.
P. Then, by cooHng, any liquid can be made solid,
and all solids become liquid when heated?
M. Generally. If substances do not decompose they
behave in that manner. Only there are liquids the freez-
ing-point of which is very low, and solids which melt at a
very high temperature. There are melting- and freezing-
points in all regions of temperature.
P. Why does a solid freeze at a definite temperature?
FORMS. 59
M. That is another senseless question. You can only
ask : What is the freezing-point connected with ? It is
just as if you were to ask: Why are there camels?
Whereas one can only ask: What properties have these
animals, and how do these properties compare with those
of other animals? In the same kind of way, melting-
points are phenomena of nature, and have definite rela-
tions to other phenomena.
P. What sort of relations?
M. If I were to answer that question you wouldn't
understand, for you would first have to know those
other properties.
P. Yes, that is true. I see you would require to
know the other properties before you could compare
relations between them.
M. Yes. So we must begin our work by collecting
facts, by writing them down, and then by comparing
them with each other in order to find out in what they
agree. That is how laws of nature are discovered.
P. I never thought of it in that way. I supposed
that some very clever man must have discovered them
all by himself.
M. Nobody does anything all by himself, as you
call it. But think a minute. One law of nature tells
us how certain things will behave under definite condi-
tions. The thing must be known under those condi-
tions before such statements can be made.
P. Yes, that is so ; but then every one must be able to
discover laws of nature.
M. And so any one can, if he finds things in conditions
which have not yet been sufficiently investigated. But
that is rather difficult, because the common and ordinary
conditions of things are already discovered ; and it is very
6o CONyERSATIONS ON CHEMISTRY.
hard to acquire enough exact knowledge to find undis-
covered spheres to examine. For instance, it would be
quite easy to discover the north pole if you could only
get to it. The difficulty is not to see the north pole, but
to get a place from which it can be seen.
P. Then I will really learn thoroughly, and perhaps
later I may discover something.
M. Yes, do so. You know the end in view, anyhow.
But now we will return to our subject. Do you under-
stand now the meaning of the name forms?
P. Yes, solids have forms, but liquids haven't.
M. That is partly right; but what about gases?
P. They haven't any form, either.
M. How do they differ from liquids?
P. They are far lighter and thinner.
M. Yes, that is right, but you haven't come to the main
point. If I pour some liquid into a vessel, it falls to the
bottom, and fills the vessel according to the amount.
But if I put gas in an empty vessel, what happens
then?
P. I don't know; a gas can't be seen.
M. It fills the whole vessel, however much or httle
there is.
P. That is extraordinary. How do you know that?
M. Only a definite amount of any sort of liquid can
be poured into a given vessel, that is, as much as there is
room for. If less is put in —
P. A part of the vessel remains empty.
M. Right. If you attempt to put more in, it doesn't
work, for a liquid doesn't allow itself to be pressed together,
or, to be exact, only slightly. A great quantity of gas can
be put into a given space, and it is always possible to put
in still more.
COMBUSTION, 6l
P. Does that go on forever?
M. No; more and more pressure is needed for it.
We shall soon go into these things more particularly.
At present the difference between liquid and gaseous
bodies is important to us. It is true that liquids have
no definite form, but they have a definite space or vol-
ume, which is unchangeable whatever form they may be
made to take. So a litre of petroleum is always a litre,
whether it is in a can or a jug, or anything else it may
be kept in.
P. And gases?
M. Gases have neither a definite form nor a definite
volume, but spread themselves out through all the avail-
able space until they entirely fill it.
P. Then the name "form" isn't suitable for gases?
M. Not at all. Liquids take the form of whatever
contains them, but only so far as they fill it. Gases
take the form of whatever contains them, because they
always fill it completely.
P. Then "form" is the way in which bodies take
their form!
M. You may describe the word in that way.
10. COMBUSTION,
M. Now you know something more about all
the three states, and can have a more complete idea
that nearly all substances are known in these three
forms.
P. Why not all?
M. With some the melting- or boiling-point is so high,
or the freezing-point so low, that it is not possible to
reach them,
62 CONVERSATIOl^S ON CHEMISTRY,
P. Oh, I wanted to ask you about that a long time
ago : are these changes of one form into another chemical
or physical reactions ?
M. You know that such a classification is more or
less arbitrary. If we define a chemical process as one
in which most of the properties of the substances con-
cerned alter, then we must consistently define change of
state as involving a chemical process.
P. But we spoke about melting and boiling in my
Physics lesson, so they belong to physics.
M. Ice can be changed as easily into water as water
into ice. But in chemical changes, only one change,
generally, is easily made; the other causes great diffi-
culty. Formerly, because of this difference, people did
not consider change of form as a chemical change.
P. You said "formerly"; is it different now?
M. Now people have learnt that many changes which
are generally called chemical can be reversed, and are
subject to the same laws as physical changes. — But
now we will turn to things which have always been
looked upon as chemical. Have you ever looked at a
candle burning? Yes? Then describe to me what you
saw.
P. When you light a candle it burns down till it is all
gone, and during this it has a hot, bright flame.
M. Right. What is necessary for burning?
P. Well, the candle.
M. Nothing else?
P. Not that I know of.
M. If you put the burning candle in water —
P. It goes out.
M. Why? What is different from before?
P, It has no more air,
COMBUSTION,
63
M. Right. For burning, then, it is necessary to have
the candle and air. I will show you now that a candle
can burn under water if it is
only put under together with air.
I let a little board float about on
the water in this large glass,
place a bit of burning candle
on it, turn a glass upside down
over it, and now I can dip the
whole thing under; the candle
is burning (Fig. 11).
P. Oh, that is pretty! Please
hold it a little longer like that.
Ah! the flame has gone out.
Some water must have got on
the wick.
M. We will make the experi-
ment again, and hold the glass quite still.
P. The flame went out again, after it had burnt a
little.
M. Now we will leave out the water altogether. I
put the little candle on a smooth plate of glass and place
a glass beaker firmly over it.
P. The flame is going out again.
M. What- must you conclude from this experiment ?
P. That the candle can't burn for long in a glass
beaker.
M. That would not be right. I put my beaker up-
right, and put the candle in. You see it burns, perhaps
a little unsteadily, but it doesn't go out.
P. Cover it with something. May I? Do you see
now the flame is out again.
M* How can you express that knowledge?
Fig. II.
64 CONVERSATIONS ON CHEMISTRY.
P. A candle can only burn for a short time in a covered
glass.
M. Must it only be a covered glass?
P. I don't think so.
M. It needn't be any one thing. An extinguisher
puts a candle out, as you know, and it is made of metal.
But why does a candle burn in a lantern?
P. Because it has air-holes.
M. What have they to do with it?
P. Fresh air always comes in at them, and the used-up
air gets out at the top air-hole.
M. Now just try to put together all that we have
spoken about.
P. The candle requires air to burn. In a closed space
a candle can burn only a short time. If the air in this
space is changed, the candle can burn longer.
M. Good. But this room is a closed space, and yet
the candle can burn here as long as it will last.
P. Yes, because the room is so big.
M. There you have discovered something. So you
think that the larger a closed space is the longer a candle
will burn in it?
P. Yes.
M. It is so. But there are many important conclu-
sions to be drawn from it. Can you give me any reason
why that is the case?
P. No.
M. We will look for resemblances. A short candle
will only burn a short time, a long candle, a long time.
Why?
P. Because in burning the candle gets used up.
Should the air get used up by burning?
M' Look for a moment. I have here a candle
COMBUSTION.
6S
attached to a wire, and lower it, burning, into a bottle
(Fig. 12). When it has gone out I
take it carefully out and light it.
If I put it at once in the bottle
again —
P. It goes out immediately.
M. It follows from that that the
air in the bottle is used up.
P. How? There is some there
still.
M. That isn't air. Air has the
property that a candle can burn in
it. What is in the bottle has not this
property.
P. But it looks just like air.
M. Quite so; what is in there is
a colourless gas like air, but not
what we call air. A chemical change
has taken place with the air, and it has other properties.
P. Other properties? Yes, the candle doesn't bum
any longer. But beyond that I don't see any other
properties.
M. That is because nearly all gases look very much
like each other. The difference of their properties
is only brought to light after careful searching. I have
here a large flask with ordinary mortar shaken up with
water and left for a while. Most of the mortar has sunk
to the bottom, but a little has dissolved in the water.
It seems also as if the water had kept its properties;
it doesn't look any different. But still it has changed.
Taste it!
P. How unpleasant! lik^ soap! It isn't poisonous?
M. No. I pour some of the lime-water into a bott)e
Fig. 12.
66 CONyERSATlONS ON CHEMISTRY.
Vfhich has ordinary air in it, and shake it. What do
you see?
P. Nothing much.
M. The Hme-water remains unchanged. Now I do
the same with the bottle where the candle burnt.
P. The water is becoming quite milky.
M. So you see that the gaseous contents of the bottle
in which the candle burnt has a property that ordinary
air doesn't possess. The air then has gone through a
chemical change.
P. So you can see by means of lime-water what you
can't see with your eyes?
M. Yes. If we could see the new things in the air
without help, we wouldn't need to make use of lime-
water. A substance, which in such a way makes known
something present, is called a reagent, and the event
which is called forth by it, a reaction.
P. A reaction means that one thing acts on another ?
M, Yes; the changed air and lime-water work upon
each other, and so the white substance is formed
which makes it cloudy. But now we will go still deeper
into the subject. What happens to the candle by
burning ?
P. It vanishes.
M. Do you mean it goes quite away?
P. Yes. Nothing remains over from it.
M. But if your book or your apple goes away, then
you ask where they can be. And in the same way
with everything else.
P. Yes, they can't vanish.
M. But the candle?
P. H'm! But" where can it be? It really vanished
before my eyes.
COMBUSTION. 07
M. Yes, it became invisible. Can't it have changed
into something invisible?
P. There isn't anything invisible.
M. Oh! isn't there?
P. No, there aren't any spirits or ghosts.
M. It seems that even they are sometimes visible.
But can you see air?
P. No. But air is changed by burning. I can't
understand that.
M. It is quite simple. The candle and air transform
each other mutually by burning, and a gaseous substance
is the result, which, owing to this state, cannot be seen.
P. Gaseous substances that aren't air?
M. So that is your difficulty? You know that many
liquids look like water that aren't water. In the same
way there are many gases that
look like air, but are something
quite different. On that account
in the earlier developments of
chemistry there was great diffi-
culty, till characteristics like that
with lime-water taught people to
distinguish between the different
gases. But now we will try another
experiment. I light a candle again,
and hold a large empty glass over
it (Fig. 13). What do you see?
P. The glass is becoming cloudy,
as if it had been breathed on.
M. What is the cloudiness that ap-
pears on a glass when breathed upon ?
P. I know that; it is made by drops of water which
come from warm breath, and are laid on the cold surface.
68 CONIFERS A TIONS ON CHEMISTRY.
M. Right. What appears in the glass consists also
of drops of water.
P. How do they get there?
M. The candle in burning changes itself partly into
water.
P. That is extraordinary! I never thought of that.
But water won't make the lime-water cloudy?
M. No, water never does that. Two new substances
are formed when a candle burns. One is water and
the other is that which makes lime-water cloudy.
P. What is it called?
M. Carbonic acid gas.
P. That is a funny name. What does it mean?
M. You can find that out later on.
P. The whole thing is becoming more and more mud-
dhng!
M. You are right. We will examine simpler cases
first; if you understand them, you will understand
the others. We are going to burn iron.
P. Can you?
M. Quite easily. You know what iron filings are?
P. Yes, they are little specks of iron which have
fallen down in filing.
M. I sprinkle some iron filings in the flame.
P. How pretty! Just like Httle stars!
M. That is burning iron.
P. Why didn't the iron gauze burn when I held it
in the flame?
M. It wasn't hot enough, for the heat is conducted
off by the gauze. But the little iron specks, on the other
hand, are quickly heated, and lose none of their heat
by being conducted off.
P. Then large pieces of iron ought to burn, if they are
made hot enough.
COMBUSTION.
69
M. And so they do. Later on we will burn iron gauze
itself. Iron burns also on melting, if it is glowing. The'
burnt iron breaks off with hammering.
P. But you don't see any flame.
M. There is such a thing as burning without flame.
The little stars from the iron filings were not flames.
We will make an experiment like that now. This black
powder is also iron, only in far smaller pieces than ordi-
nary iron filings. I put a small wire tripod on the bal-
ance, lay a narrow piece of wire gauze on it, and shake
out several grams of iron powder on it (Fig. 14).
The whole thing is now made to balance. Then I hold
Fig. 14.
a flame to the edge of the little heap. Now it is begin-
ning to burn.
P. I only see it glowing.
M, That is how powdered iron burns. Charcoal can
only glow when it burns.
P. That is true. But why have you put it all on a
balance ?
70
CONVERSATIONS ON CHEMISTRY.
M. You will soon see. What do you think? will
iron become lighter or heavier by burning?
P. I should think lighter. The scale with the iron
will rise.
M. Notice carefully.
P. It is sinking! Perhaps it was only a draught. No,
it is getting heavier. That is very odd.
M. Why?
P. One time a thing becomes lighter by burning,
another time heavier.
M. In the case of the candle the thing that was made
by burning went away, with iron it remains. If it stays,
it increases the weight.
P. With the candle as well? I'd like to see that.
M. To do that, you have only got to keep hold of what is
made by the candle burning — water and carbonic acid gas.
P. That must be rather difficult.
M. Not very. There is a sub-
stance called caustic soda which has
the property of binding together
every trace of water and carbonic
acid gas with which it comes in
contact. I put some loose pieces of
it in the top part of a lamp-funnel,
which I place over a burning
candle (Fig. 15), and put the
whole on the scale and balance
it. We won't need to wait very
long.
P. Yes, the scale with the
candle is beginning to sink.
M. And the proportion will be more, the more the
candle is burnt.
Fig. 15.
OXYGEN. 71
P. Is it the same with all burning substances?
M. Yes ; you may try to burn, after this, oil, petroleum,
or matches, or whatever else you like, instead of a candle,
under the cylinder with caustic soda. The weight will
always be increased.
11. OXYGEN.
M. What did you learn last time?
P. That all bodies become heavier by burning.
M. That is not quite complete. Think of the candle.
P. That all bodies become heavier by burning if
you take w^hat is formed into account.
M. Think of the candle again! If it was entirely
burnt ?
P. Ah, yes! That which is formed by bodies in
burning is heavier than the body was itself.
M. Now it is right.
P. But can iron burn, too, so that nothing remains?
M. So that no iron remains? Certainly. Look, for
yourself, what the iron powder that we burnt yesterday
has become.
P. It is a dark mass, which looks rather like the
powdered iron, only it is caked together.
M. Take some, and crush it in the mortar.
P. There is a black powder.
M. Now grind some powdered iron after you have
cleaned out the mortar.
P. It is bright like iron.
M. Now you see the difference. The burnt iron
isn't iron any longer, but a substance with other proper-
72 CONVERSATIONS ON CHEMISTRY.
ties, and the iron has vanished in the same way that the
burnt candle vanished.
P. But the air, which helped with the burning?
M. The same thing happened to it which happened
to the iron. In the same way as the solid iron changed
to the solid substance, smithy scales, the vanished part
of the air used by burning a candle becan^e part of
another gas.
P. Has another gas been made by iron as well?
M. No.
P. Then air must vanish when iron burns in it.
M. We will make the experiment. I put my tripod
with powdered iron on a little floating board, hght it,
and cover it with a large glass, so placed that it stands
on the bottom (Fig. i6). As the experiment is rather
slow, we must wait till the glowing iron is extinguished
and has become cold. What do you see now?
P. It really seems as if air had vanished; but only a
part, less than a quarter.
OXYGEN. 73
M. If you measure it more closely, it is about a fifth.
P. Perhaps you took too Httle iron.
M. No. If I had taken morC; no more air would have
been used. -
P. But this is quite different from a candle, and also
from iron. You can burn them completely.
M. Can you bum wood entirely?.
P. Ash remains.
M. It is the same with air. Wood is a mixture of
combustible and non-combustible substances; when the
former are burnt, the latter remain behind. Air is a
mixture of two gases ; the one gets separated by burning,
and is called oxygen, the other is left unchanged, and is
called nitrogen. Oxygen only takes up about a fifth
part of the space of air.
P. If you had pure oxygen, would it entirely vanish
in burning?
M. Certainly, if there were no other gas. We will
make pure oxygen.
P. Can we?
M. It has been possible for rather more than a hundred
years. This white salt is called potassium chlorate.
When it is heated a great deal of oxygen is formed.
P. What sort of brown powder are you mixing with it ?
M. That is heated iron-rust. When some is put with
it, the oxygen forms more easily and regularly. I
shake the mixture in a Httle round flask. Now I must
make my apparatus. To do that, I take a cork which
just fits into the neck of the flask, and cut a piece of
glass tubing.
P. How can you cut glass?
M. It isn't exactly cut, but broken. But so that the
fracture comes at the right place, and is even all round,
74
CONVERSATIONS ON CHEMISTRY.
I must scratch, and divide the tube at the place I
wish.
P. What sort of tool is this?
M. This is an old three-cornered file whose teeth
are ground down so that three cutting edges are made.
If I saw the glass tube with this sharp edge, it will crack.
Then I break the tube apart with the crack furthest
from me, so that it breaks off evenly (Fig. 17).
Fig. 17.
P. That is clever! Can I do it too?
M. I will give you a piece of glass tube afterwards
and you can practice on it. Now I will bend the tube.
P. You can't; it will break.
M. Heat will make glass soft, so that it can be bent.
I put the part where the bend is to come in the flame
and turn the tube continually, so that it shall be evenly
heated, otherwise it would crack. After a time the
glass will become so soft that it will bend with its own
weight. I help it a little till it is the right shape, and let
the glass get cold and hard, then it will keep its new shape.
P. That looks so easy. Can I do it too?
M. It isn't difficult, but it needs practice. The main
thing is, not to apply the heat to any one point, and
only to use very light pressure in bending it. Otherwise
OXYGEN. . 75
the bend may easily come uneven. Now the other end
of the tube must be bent a Httle, and finally I turn each
end in the flame for a little, so that the sharp edges become
round and can't cut or scratch any more. That must
never be forgotten.
P. How is that managed?
M. Soft glass behaves like liquids. You know that
instead of having corners or points, their surfaces are
always rounded.
P. Why do liquids do that?
M. On account of the surface tension. Through this
the surface endeavours to become as small as possible,
and as a ball is the form which contains the most contents
with the least surface, all liquids try to form themselves
into the shape of a ball.
P. But liquids take the form of the vessel that contains
Inem.
M. Quite right. This comes from gravity, as they
always try to get as low down as possible. Both causes
work simultaneously on the liquid, but the gravity is
generally far the stronger, and the shape of the water is
most dependent on that. Now we must make a hole in
the cork. For that I bore a hole with a steel point, an
awl, and then I file it with a somewhat large round file till
I can with a little force stick the glass tube through.
Now everything is put together, and I fasten the
apparatus, so that I can slip a piece of wire gauze below
the flask and put the lamp under it (Fig. i8).
P. Why do you put the end of the tube in a dish of
water ?
M. To collect the gas. If I were to stick the tube in
an empty flask, that is, one filled with air, and were tc
lead the gas into it, it would mix with the air, and I
76
COhiyERSATlONS ON CHEMISTRY.
shouldn't be able to see when the flask was full. So
I fill the flask with water, and allow it to be displaced by
the entering gas, holding the mouth of the flask over
Fig. i8.
the glass tube. As the gas doesn't mix with water, I
get it pure.
P. Bubbles of gas are rising; hold the flask over them!
M. At first it is only the air which was in the appa-
ratus.
P. Then how do you know when the new gas comes?
M. I take the tube out of the water and hold a glowing
splinter at its open end. What do you see ?
P. It goes on glowing.
M. Then it is only air. And now?
P. Oh, it begins to burn of itself!
M. Not of itself, but with the oxygen which comes
out. Now I bring the tube below the water again, and
hold the flask over it. But in order not always to have
to hold it, I place it in a little lead stand, below which
the tube ends, so that the bubbles rise in the flask
and expel the water (Fig. 19). In the mean time I will
OXYGEN.
77
fill some flasks with water so that we may afterwards fill
them with oxygen.
P. Please show me the ex-
periment with the glowing splin-
ter again.
M. It is the test for oxygen.
Whenever a glowing spHnter is
put into oxygen it catches fire.
I can repeat the experiment
often with the oxygen in this
flask. But at last it gets used
up, and the experiment won't
succeed any more.
P. What is the reason of
that? ^iG- ^9-
M. I will first show you some other similar experiments.
I tie a piece of charcoal on to a wire, light it at one corner,
and place it in the oxygen. It soon
begins to glow all over, much more
brightly than in air. A bit of sul-
phur in a little iron spoon which
you can hardly see burning in the
air gives a bright blue flame. A
piece of phosphorus in a similar
spoon, which burns in the air with
a yellow flame, looks as bright as
the sun. A thin spiral of iron wire,
on the end of which a little tinder
has been fastened and made to
glow, catches fire and burns,
throwing out sparks, and the
smithy scales fall, while hot, into the water which covers
the bottom of the flask. It is better to put a little sand
Fig. 20.
78 CONVERS/iTIONS ON CHEMISTRY.
at the bottom of the flask so as to keep it from breaking
(Fig. 20).
P. Oh, that is a beautiful firework!
M. We won't forget what the firework means. What
can you say in general about these experiments?
P. Things burn much more brightly in oxygen than
in air.
M. Quite right. But they burn in air too, at the
expense of the oxygen that is present. Where does the
difference lie?
P. They give off more heat when in pure oxygen.
M. Your answer is right or wrong according to what
you mean by the word heat. If you mean to say that
the quantity of heat that i gram of carbon or iron
gives out is greater when it burns in oxygen than when
it burns in air, that is wrong. The quantity of heat
is the same. But if you mean that the temperature rises
higher, that is right.
P. I mean the temperature.
M. Of course you do! This is the reason. The same
amount of heat which is given out in both cases has
only to heat the resulting product of combustion when
the substance burns in pure oxygen; but when it burns
in air it has to heat the nitrogen which is mixed with
the oxygen.
P. Is the brighter light connected with the higher
temperature ?
M. Certainly. The temperature can even be estimated
from the brightness of the light. But, besides that, the
higher temperature makes the burning take place more
quickly.
P. What has that got to do with it ?
M' It has been generally found that chemical changes
OXYGEN. 79
take place more quickly the higher the temperature.
But we will go back to oxygen again. The phenomena
you have seen are all chemical changes, for the burning
substances and the oxygen have disappeared, and new
substances have been produced instead.
P. Are the heat and the light which have been pro-
duced, new substances too?
M. No, these things are not called substances, because
they possess neither weight nor mass.
P. But all the same they are really there.
M. Certainly, because they are real things. They
behave something like substances, for they change into
one another, and new quantities of them can never be
produced except by such change. Only they have no
weight like substances.
P. Then these must be what are called forces?
M. People used to call these things forces, but that
led to a misunderstanding, since the word force had
already been used for something different. Now they
are called energies. Heat is one kind of energy, and
light another.
P. Yes; people are said to be energetic who can do
something and carry it through.
M. The scientific use of the word energy is pretty
nearly the same. Energy is what causes things to
change.
P. So when substances change by chemical action,
is that energy too?
M. To be sure; only we express it somewhat differ-
ently. We say that substances possess chemical energy
when they are able to act on each other and produce
new substances. At the same moment that the substances
change, a change of a part of their chemical energy
8o CONVERSATIONS ON CHEMISTRY.
takes place, and this assumes the form of heat or light,
and sometimes of electrical or mechanical energy.
P. That strikes me as very curious and mysterious.
M. The change of one kind of energy into another is
not more mysterious than the change of one substance
into another; indeed, it is even simpler. To tell you a
little more about energy : you must know that the ordinary
work which a man or a horse or a steam-engine does is
also energy.
P. Then I ought to be able to make heat or Hght or
electricity by my arms.
M. So you can; when you rub your hands together
they grow warm, and if you turn a blunt drill with all
your force in a hole, it soon becomes so hot that you
could burn your finger with it. And you know already
that people can make fire by friction.
P. Yes, that is true. So I can make as much heat
as I like?
M. Not as much as you like, but as much as you can.
When you turn the drill for some time you can't go on
any more; you are quite exhausted; that is, you have
used up the store of energy which you possessed.
P. Where did I get that energy from?
M. From your food. It is chemical energy which
you have taken in with your food, and in your body
there is a kind of apparatus, the muscles, which change
chemical energy into work.
P. How do they do that?
M. I wish I knew! Investigators haven't found out
yet how it is done. But there is no doubt that chemical
energy is used up in doing work, for you see that a hard
worked horse must be well fed in order to do its
work.
OXYGEN. 8i
P. But I always have a good appetite, even when I
do nothing.
M. Then you waste the chemical energy of your food.
Of course you always want a certain quantity in
order to keep your temperature up to 37° C; for as
your body is considerably warmer than its surround-
ings, it is continually losing heat which must be replaced
by means of food. That is a second way in which you can
make heat, although it is beyond your control.
P. Can I make light too?
M. Yes; if you rub two pieces of sugar together in
the dark, they will make light.
P. Don't they make light in the daytime?
ikf. Yes; only the Hght is so weak that it can't be seen
by daylight. This experiment shows that the work of
your muscles can be changed into light.
P. But can't I make light without anything?
M. You can't; but glow-worms and little animals,
which are the cause of the phosphorescence of the sea,
can. These change the chemical energy of their food
directly into light.
P. And can I make elec^trical energy too?
M. Certainly ; you have only to rub a piece of sealing-
wax with a cloth.
P. Oh, yes, I know. But I must use my muscles again
to do that ; I don't do it directly.
M. Electric currents run through your body when-
ever you exert yourself, indeed whenever you think.
But they stay in your body and it isn't easy to conduct
them outside.
P. I never dreamt that I could do all these things!
M. Well, you needn't be conceited about it, for every
animal can do the same.
S2 CONyERSATlONS ON CHEMISTRY.
P. Still, it's very queer. Where does the energy of
food come from?
M. From the sun.
P. I don't understand that.
M. Where does food come from? Either from plants
or animals. Plants grow only in sunlight, for they use
the energy of hght to build up their structures; they
store it up in this form. And we consume the energy
of the sun in the plants. And the animals whose flesh
we eat subsist upon plants, that is, upon the energy of the
sun.
P. I shall think of the sun quite differently after
this.
M. If you only think of what we have been speaking
about, you will understand more of the world than you
did.
12. COMPOUNDS AND CONSTITUENTS.
M. Last time you learned a great deal that was new
to you. Tell me the principal things.
P. First I learned how oxygen is made and collected;
then I learned that substances burn far more brightly in
it than in ordinary air, and that is because air contains
only a fifth part of oxygen. Then I learned something
about energy. But that was so much and so unfamiliar
that I can't say it in a few words.
M. We will try together. Wherein does energy resem-
ble substances, and wherein does it differ from them?
P. Resemble? Yes, it can change into many kinds,
and when one kind is formed, others vanish.
M. Right. Where do they differ?
COMPOUNDS AND CONSTITUENTS. ^Z
P, Energy can't be weighed, and it comes from the
sun on to the earth. Substances don't come from there.
M. No, at any rate not in detectable quantity.
Now, first of all, be sure of these points; the others will
be much more comprehensible if we are careful over
these. Now we will return to oxygen. There is still
here a flask which was filled yesterday. What noticeable
properties has it?
P. Oxygen looks like air; it is colourless.
M. What does it smell like?
P. I can't smell anything; it has no smell.
M. You should have been able to tell me that without
opening the bottle. Just think, a fifth part of the air is
made of oxygen.
P. Oh, yes, because air doesn't smell, oxygen can't.
M. These are the noticeable properties of oxygen.
Besides these it has others, which can only be found out
by measurements or experiments. The burning phenom-
ena that I showed you are also such properties. They
are called chemical properties, because they depend upon
chemical processes. Also the reaction of oxygen, the
bursting into flame of a piece of glowing wood, is one
of these chemical properties. Now we will learn another
way of telling oxygen. This brick-coloured powder is
called mercuric oxide. I put some into a test-tube made
out of a particular kind of glass that has rather thicker
sides, and is more difficult to melt than ordinary glass,
and attach a gas-delivery tube as before. Then I make
the glass hot with a lamp. What do you see?
P. The red powder is becoming black. It is becom-
ing charred.
M. No; if I let it get cold it will become red again.
P. Then how does it get black?
84
CONVERSATIONS ON CHEMISTRY.
M. There are many substances which change their
colour with heating. Colour depends to a great extent
on temperature.
P. Now there are bubbles coming (Fig. 21).
Fig. 21.
M. Again it is, first of all, air which has expanded by
heat.
P. But now the bubbles are much more frequent and
continuous.
M. We will collect some of the gas in a little test-tube
and try it with the glowing splinter. It is still air that
is coming out of the test-tube. But the second time it is
filled—
P. The splinter has caught fire; it is oxygen.
M. Perhaps. We will collect some and see if it is
colourless and scentless. Try it!
COMPOUNDS AND CONSTITUENTS. 85
F. Yes, it is scentless, and one can't see any colour.
But why was it necessary to prove it in this way as well ?
M. Before one can say that anything that one has is
a definite substance, one must have made sure that
all its properties are the right ones.
P. But then, one can't look for all its properties; there
would be no end to it.
M. There you are right. But several properties must
always be tested for, because it often happens that dif-
ferent substances have one common property, whereas
other properties are different.
P. Really just the same property?
M. One can't be absolutely certain, even when no
difference can be seen. Since no property can be noticed,
or measured with absolute exactness, one can't be certain
that a seeming resemblance will not turn out to be a
difference on closer inspection. But just to make these
difficult researches unnecessary, people examine several
properties. For it is very seldom that two different
substances have several properties in common.
P. Look what has happened to the experiment mean-
while. The test-tube looks just like silver at the top.
M. Yes, and the greater part of the mercuric oxide
has disappeared. I heat it a little longer and now
it is all gone. I take the delivery-tube out of the water
and let everything cool.
P. Why don't you leave it all as it is?
M. The hot oxygen gas in the tube in cooling would
contract, and the water might enter the test-tube. Now
look closely: the silvery stuff in the tube can be brushed
together with a feather, and changes into bright liquid
droplets.
P. They look just like mercury.
86 CONVERSATIONS ON CHEMISTRY.
M. They are mercury.
P. But how did it come there?
M. It came out of the mercuric oxide.
P, And has the oxygen been made from it too?
M. Yes, these two substances, and nothing else.
P. But why isn't the mercury where the mercuric oxide
was?
M. Because the mercury with the heat from the lamp
became volatile, that is, it changed to a vapour. Then,
when the tube was colder, the vapour changed back
again to liquid mercury. I will now take some more
mercury in a test-tube, and heat it; look, the first drops
are forming, it is becoming thicker, and now it looks like
a silver looking-glass. I repeat the experiment with
the liquid metal that I made before; you see it behaves
just the same; it is mercury too. But take care of the
vapour; it is poisonous.
P. I shouldn't have thought so!
M. Why not?
P. Mercury is a metal, and metals don't boil.
M. Certainly they do, only the boiling-point of most
of the best-known metals lies so high that it can't be
reached by ordinary means. But, for example, in the
flame of the electric arc all known metals turn to
vapour. Mercury, however, boils fairly easily at 350° C.
But now we will go back to our experiment. You saw
that by heating, the red powder changed into mercury
and oxygen. Out of mercury and oxygen red mercuric
oxide can be made again. You can, so to speak, reverse
the reaction.
P. That is wonderful. Can I see it?
M. Unfortunately I can't show you. Mercury oxide
is fonned out of mercury and oxygen, if they are left in
COMPOUNDS AND CONSTITUENTS, ^7
contact together at a temperature something over
300°. But that takes place so slowly that it would
take a week to get a couple of grams. But if it is
done it shows exactly the same properties as mercury
oxide.
P. Isn't it made in that way, then ?
M. No, it is made in quite a different way, which you
wouldn't understand yet.
P. Then it doesn't matter which way it is made?
M. Certainly; there is an important general law, that
a definite substance always has the same properties in
whatever way it may have been made.
P. I shouldn't have thought so!
M. You have just had an example of it: the oxygen
made from mercuric oxide had exactly the same properties
as that made from potassium chlorate.
P. Yes, so it did. I never thought of that; I took it
for granted.
M. You see again: People take things for granted
when they don't think about them. Now notice some
new names; because from one single substance, mer-
curic oxide, two different substances, mercury and
oxygen, can be made, and vice versa; from the two
latter, again, a single substance, mercuric oxide, can be
made; the latter is called a compound and the former
the constituents. So mercuric oxide is — ?
P. Mercuric oxide is a compound of mercury and
oxygen.
M. Yes, and mercury and oxygen are the constituents
of mercuric oxide. — Now we are coming to an important
question about the proportions by weight in chemical
processes. In this closed flask there is oxygen, and there
is a piece of charcoal hanging from a wire in it. I weigh
88 CON VERS ATIOhlS ON CHEMISTRY,
the flask on the balance. Now I will light the charcoal
without opening the flask.
P. How will you do that?
M. I could do it in several ways. If I had put a
second wire through the stopper, and bound both wires
together with a thin piece of iron wire, I could make it
glow with an electric current, and it would light the
charcoal. But since we have sunshine, I can do it in a
far simpler way : I shall light the charcoal with a burning
glass.
P. Good; that's splendid. Hurrah! the charcoal is
burning already.
M. And now it has gone out again, since the oxygen is
used up. Now what do you think; will the flask have
become heavier?
P. Of course.
M. You have taken it for granted again! But we
will look. What do you see?
P. The pointer is going backwards and forwards over
the middle. The weight seems to have remained the same.
Perhaps the increase is so little that it can't be noticed?
M. No, even with the most careful weighing it would
always be the same.
P. But that can't be right! I learned and saw that
weight increased with burning.
M. The weight of what?
P. Ah! so it was. The product of combustion
weighed more than the burnt body weighed.
M. Well, and here?
P. Here it weighs the same.
M. That is a false conclusion. It really weighs more.
P. But then, how is it that the weight didn't change?
M. It is because the oxyen disappeared. The product
COMPOUNDS AND CONSTITUENTS. 89
of the burning weighs just as much more as the used
oxygen. So the gain and loss have balanced each other.
P. That is extraordinary.
M, Yes, it is an example of one of the most important
laws, which holds for all chemical, and also for all physi-
cal, processes; whatever changes take place between defi-
nite substances, they never change their combined weight,
P. But the separate weights change?
M. Certainly; but what the one side loses, the other
gains. The law only refers to the sum of all the weights.
P. You always taught me, in cases like this, never to
ask why it is so, but with what it is connected. Is any-
thing known about it?
M. Certainly. You know that weight and mass are
in every place proportional. So also the law of the
unchangeableness or conservation of mass holds.
P. What is the use of this law?
M. It makes it possible to account for proportions by
weight in chemical changes even when you cannot, or
do not want to, weigh each substance separately. For
example, if I weigh the amount of mercuric oxide I
take, and the amount of mercury I get from it, then I
know how much oxygen was there too. Because there
must always be this equation: Mercuric oxide = mer-
cury + oxygen, where the name of the stuff denotes its
amount by weight.
P. Has oxygen a weight? It is a gas!
M. Do you think that gases have no weight?
P. I can't believe it.
M, The density, or the relation of the weight to the
volume, is small with gases, several hundred times smaller
than with water. But they certainly have weight. One
litre of ordinary air weighs more than one gram.
90
CONVERSATIONS ON CHEMISTRY.
P. I'd like to see that.
M, I can show you quite easily. Here is a flask of
strong glass which I close up with a stopper, in which
there is a glass stop-cock. So that it shall not get pulled
out, I tie it firmly down with wire or string. Now I
shall weigh it all. I can pump air into the flask through
the open stop-cock with a bicycle pump. After pump-
ing twice, I close the stop-cock, put the flask again on
the balance, and it has become distinctly heavier.
P. Can you see how much air you have pumped in?
M. Yes; with the aid of a Httle rubber tubing I con-
nect the dcHvery tube from the oxygen apparatus with
the stop- cock, put a flask filled with water over the end,
and now, if I open the stop-cock, the air which I pumped
in will come out, and collect in the flask (Fig. 22). If you
Fig. 22.
had weighed the flask exactly before, and weigh it again
now, the loss of weight is the same as the air that has just
come out. And if you know the capacity of the flask,
you can measure the volume of the air.
COMPOUNDS AND CONSTITUENTS. 91
P. Yes, so you can!
M. After this you may try to measure like this; you
will find that air is about 800 times lighter than water.
Now we will go back to our experiment. Have you
noticed the amount of oxygen I got from potassium
chlorate and mercuric oxide?
P. Yes. There seemed to be far less from mercuric
oxide.
M. Yes. One gram of potassium chlorate gives far
more oxygen than one gram of mercuric oxide. But if I
make the experiment twice, each time with one gram of
mercuric oxide, what will be the result?
P. Each time it will be the same.
M. And with potassium chlorate?
P. The same.
M. You think, then, that if a substance is changed
into another, this always happens according to definite
proportions by weight.
P. I don't know whether it is quite definite, but it
must be so, more or less.
M. It is exactly so. You could have thought of that
before. For a definite substance has always quite defi-
nite properties; its capacity, in certain cases, to change
into another substance, is one of these properties; it
follows that the ratio of the weights of the original stuff
and of the product of change must be definite.
P. I should never have had the courage to draw such
a conclusion.
M. How can it be proved that such a conclusion is
right?
P. By experiment.
M. Right. Now experience has shown for several
hundred years that between the substances which take
92 CONyERSATIONS ON CHEMISTRY.
part in any change, roughly speaking, a definite ratio
must exist; from a pound of fat an unhmited amount
of soap cannot be made, but somewhere about the same,
and so on. But it is only in the last one hundred
years that this question has been carefully tested and
the law found to be quite exact.
P. Does it apply to all substances?
M. To all pure substances; that is, to those that
are neither solutions nor mixtures.
P. It is strange. The laws which you have taught
me up till now are all very simple and easy to under-
stand. But I'm afraid I will never be able to under-
stand and use them at the right time.
M. That is only natural. A law is like a tool: if you
have had no practice, it is of very Httle use having the
tool, even if you know what it is for. But what we are
going to talk about later on will give you the necessary
practice.
13. ELEMENTS.
M. Last time you learned two important laws, which
.show the relation of the proportion by weight of such
substances between which chemical changes take place.
The one was called the law about the conservation of
weight; just say it over!
P. If chemical changes take place between given sub-
stances, the combined weight is not changed by it.
M. And what is the other law about?
P. About the proportion of weight in chemical changes.
If one substance changes into another, the weight of the
one has a definite ratio to the weight of the other.
ELEMENTS. 93
M. Right. It is called the law of constant proportion.
P. But what connects these ratios?
M. That is a sensible question ! I can give you a very
wonderful answer for it. But to do that I must first
make a new idea clear to you : that of chemical elements.
You remember the equation: Mercuric oxide = mercury +
oxygen; what sort of quantity was -concerned?
P. That of weight.
M. Now, if you split up a definite quantity of mercuric
oxide by heat, and collect the mercury, will it weigh
more or less than the mercuric oxide?
P. Let me think a minute. It must weigh less.
M. Why?
P. Because it weighs as much as the mercuric oxide,
with the oxygen, and oxygen has weight too.
M. Right. Then if mercury is made into mercuric
oxide, or oxygen into mercuric oxide, in each case weight
is added: in the one case, the needed amount of oxygen,
in the other. case, of mercury.
P. I understand that.
M. You remember also that we called oxygen and
mercury the constituents of mercuric oxide, and the
latter a compound of the former.
P. Yes.
M. Then it follows that a constituent must always
weigh less than any of its compounds.
P. Because something is added each time. •
M. Quite right. Now you can believe that all sorts
of chemical experiments have been made with oxygen,
like the one you have seen, and that every time the weight
of the new substance, which was the result of the con-
sumption of the oxygen, was determined. No single
instance has been found in which one of the resulting
94 CO^yERSATIONS ON CHEMISTRY.
substances weighed less than the oxygen it contained.
All weighed more.
P. Then oxygen can only form compounds ?
M. Yes, and no constituents of oxygen are known.
Substances of this sort are called elements. What is
an element?
P. A substance, all the products of change of which
weigh more than it does itself.
M. Quite right. It can also be said that an element
is a substance of which no constituents are known. But
this definition is not so clear, because it must first be
known what a constituent is.
P. But I learned before that an element was an unde-
composable substance'
M. It means the same thing. The changing of a
substance into its constituents is called decomposition.
Because, out of a single thing, several different ones
arise, such a process is called decomposition.
P. Now I understand. But to decompose means to
separate what is already there, not to change it.
M. It is like this: If a definite amount of mercury
and oxygen has changed, or united into mercuric oxide,
it is true that the mercury and oxygen have vanished,
but they can be obtained out of it again at any time.
And exactly the same amount of each constituent is
obtained as was originally there. You can look at it
in this way : as if both the constituents in the compound
were still really present, and had hidden themselves, as
it were, when they combine with each other. Hence
the expressions decompose and combine.
P. Yes; which is true then? Are the constituents
really in the compound still, or not?
M. You asked that question without thinking. A
ELEMENTS. 95
compound isn't a bag or box in which something can be
*'in." If you understand by "in" that by suitable
means they can always be taken out of the compound,
they are in it. But if you mean that they are hidden
away somehow in the compound with all their properties,
that wouldn't be clear and would be misleading. You
know now what I mean when I say oxygen is an
element.
P. Are there more elements?
M. Certainly, mercury is one too. Sulphur, iron, tin,
lead, and copper are also elements. There are altogether
about seventy- five different elements. Here is a table
of elements (see on the next page); if you look through
them you will see some friends. But most of them
are unknown to you. A great many of them also are
very rare, that is, the substances out of which they
can be procured are rarely found.
P. Can't the rare elements be made out of other sub-
stances which are more frequently found?
M. No, that can never be the case. A given com-
pound can only be divided up in one way into elements,
that is, from every substance only definite elements
can be obtained, and however one may try, the same
elements are always found in the same proportions.
And to make this substance artificially, just the same
elements must be taken in the same proportion,
or compounds must be made use of from which
these elements can be got, or in which they are
"contained."
P. Is that another law of nature?
M. Yes, it is the law of the conservation of the ele-
ments.
P. Please explain it a little more.
96
CON VERS A TIONS ON CHEM IS TR Y.
Aluminium Al
Antimony Sb
Argon. Ar
Arsenic As
Barium Ba
Beryllium Be
Bismuth Bi
Boron B
Bromine Br
Cadmium Cd
Caesium Cs
Calcium Ca
Carbon C
Cerium Ce
Chlorine CI
Chromium Cr
Cobalt Co
Copper Cu
Erbium Er
Fluorine F
Gadolinium Gd
Gallium Ga
Germanium Ge
Gold Au
Helium He
Hydrogen H
Indium In
Iodine I
Iridium Ir
Iron Fe
Krypton Kr
Lanthanum La
Lead Pb
Lithium Li
Magnesium Mg
Manganese Mn
Mercury Hg
Molybdenum Mo
Neodymium Nd
Neon Ne
Nickel Ni
Niobium Nb
Nitrogen N
Osmium Os
Oxygen O
Palladium Pd
Phosphorus P
Platinum Pt
Potassium K
Praseodymium Pr
Radium Ra
Rhodium Rh
Rubidium Rb
Ruthenium Ru
Samarium Sa
Scandium Sc
Selenium Se
Silicon ■ Si
Silver Ag
Sodium -. Na
Strontium Sr
Sulphur S
Tantalum Ta
Tellurium Te
Terbium Tb
ThalHum Tl
Thorium Th
Thulium Tu
Tin Sn
Titanium Ti
Tungsten W
Uranium U
Vanadium V
Xenon X
Ytterbium Yb
Yttrium Y
Zinc Zn
Zirconium Zr
M. You know that some time ago there were chemists
who gave their whole hfe trying to make gold or silver
out of lead or other cheap metals, without one of them
succeeding; they were called alchemists. Now, the
whole of alchemy was built upon the hope that it was
possible to change one element into another, perhaps
lead into gold. It could not be foretold that this was
not possible; it was only by resultless efforts continued
ELEMENTS. 97
through centuries, that it was found to be impossible
in the case of gold and silver, and, later, in the case of
all other elements.
P. Then the gold-making wasn't so mad and useless
in the long run?
M. Neither the one nor the other. It wasn't mad,
because it became known that it couldn't be done. Only
the gold-makers didn't work scientifically, that is, in an
orderly manner, because they only tried things on chance.
And the final result — that elements could neither change
into each other, nor the compounds of definite elements
into the compounds of other elements — was an important
scientific discovery, which made the study of chemistry
far easier.
P, I don't understand that.
M. Just suppose that if we provide each element with
a definite sign, then we can mark every compound by
putting the signs of their elements together. Just as
you make the word "hat" out of only the signs h, a,
and t put together, and it can only be divided up
into these signs, and you can never build up the word
"rose" out of these signs, so compounds and elements
act in the same way. In the table of elements (page 96)
there is a sign like that, against every name, that is made
from the first letter of the name, and generally a second
letter as well. Every substance that is on the earth
can be represented by placing together such signs, for
however many substances there are, every one of them
can be decomposed into elements only in its own par-
ticular way.
P. I see, it is again one of those laws which are really
very simple, only you must be accustomed to them first.
ikf. You will soon get accustomed enough to them.
gS CONyERSATIONS ON CHEMISTRY.
In the mean time we will take our table of elements and
see how much chemistry you know already from daily
life. Oxygen you know already; it is a colourless gas.
Hydrogen is a colourless gas too, but combustible.
P. What is hydrogen?
M. An element that can be obtained from water.
P. Then isn't water an element?
M. No, it isn't in the table. It is a compound of
oxygen and hydrogen. You know something about
nitrogen too; it is the other ingredient in the mixture
of ordinary air. It is also a colourless and tasteless gas.
P. Yes, because air is.
M. Right. Now comes carbon. It isn't a gas, but a
solid body. Ordinary charcoal consists of carbon, but
not in the pure state. These four elements are always
in all living things, plants as well as animals, and as
such form a definite group. That is the reason I named
them first to you. Moreover they are the type of four
different groups of other elements.
P. What does that mean?
M. Among the other elements there are a number
which behave in the same way as oxygen, while others
are more like hydrogen, others like nitrogen, and again
others like carbon.
P. What do you mean by "like"?
M. They have to some extent similar physical prop-
erties in an uncombined condition as so-called free
elements. In many cases also the compounds which
are formed with a third or fourth element are like in
their properties.
P. That doesn't appear to me a definite reason foi
classifying them.
M. Neither it is. But by taking into consideration
ELEMENTS. 99
all the properties of all the compounds which can be
produced from an element, so many resemblances and
differences turn up that a chemist who knows the rela-
tionships doesn't find the choice difficult. As you don't
know them yet you must simply accept my classification.
P. But it appears to me to be unscientific to accept
anything that I can't prove.
M. You will be able to prove it when you have learnt
enough chemistry. Besides I won't use the classification
for any scientific conclusion, but only for your own con-
venience, so that you can learn the facts more easily;
besides, such arbitrary things can be treated in science
in an arbitrary manner.
P. Yes I see.
M. Now impress the following names on your mind:
* Hydrogen
* Oxygen
* Nitrogen
* Carbon
* Chlorine
* Sulphur
* Phosphorus
* Silicon
* Bromine
Selenium
Arsenic
Titanium
* Iodine
Tellurium
Antimony
Later on we will study carefully only those elements
marked with an asterisk.
P. Why only these?
M. The others are either too seldom found in nature,
or their compounds have too little importance in their
applications. As we can't learn nearly all that has been
found out up to the present in chemistry, we must be
satisfied with a selection. I arrange this so that at any
rate you learn the substances which on account of their
uses, or on account of their sources, come most frequently
before our notice.
P. Then I am only to learn a little part of chemistry?
M, There are very few people who know every fact
that has been proved in chemistry up to the present. I
shall try to teach you those parts of chemistry that wiU
loo CONVERSATIONS ON CHEMISTRY,
give you the best conception of the most important
relations. Later you can take up a special branch,
which you can learn as thoroughly as you wish. But
now we will speak about the elements we have chosen.
I have already told you about hydrogen, that it is a colour-
less, combustible gas; but its flame is quite pale and
gives very little light. It is the lightest substance there
is, and for thai* reason is used for filling air balloons.
P. Is there hydrogen in the little red india-rubber
balloons that children play with?
M. Certainly, and if one of these freshly filled balloons
is set fire to, the hydrogen burns with a puff.
P. I will try that next time.
M, But don't hold it too near your face, or you may
burn yourself, for the flame is hot, and it often goes
off with a tremendous bang. — Chlorine is a greenish
gas, with a very unpleasant, pungent smell. Perhaps you
have already smelt it, because a white powder called
chloride of lime is often scattered on unpleasant-smelling
decomposing matter; its smell is that of the greatly diluted
chlorine.
P. Yes, I remember; our boy always strews it at the
street corner. Why do people do that?
M. The chlorine destroys the bad-smelling substances
and kills the offensive little germs or mould or bacteria.
— Bromine is at ordinary temperature a deep reddish-
brown coloured liquid, and has a yellowish-red vapour
which smells the same as chlorine.
P. Ah, then that is one of those resemblances of which
you spoke.
M. Yes. Iodine smells like it too, only at ordinary
temperature it is a solid, shiny, black substance, the
vapour of which is violet.
ELEMENTS. loi
P. I remember that my throat was painted with tinc-
ture of iodine once. Has that anything to do with the
element iodine?
M. Yes, it is a solution of iodine in spirits of wine.
With that we finish the first group. Of the second you
already know oxygen. And sulphur is familiar to you
too.
P. The yellow stuff?
M. Sulphur is a solid substance of a yellow colour,
and burns with a blue flame.
P. And in doing so gives off a very bad smell. Why
do most substances in chemistry smell so queer and un-
pleasant ?
M. The bad-smelling substances are mostly those
which have a corrosive effect upon the inner skin of
the nose. If they didn't smell badly, we wouldn't notice
anything, and we would always have a sore skin and a
cold in our noses. Chemistry would be a far more
dangerous thing to work with than it now is.
P. Ah, that is good. Do all poisonous substances
smell nasty?
M. First of all, we can only smell those substances
that change into gas or vapour, because otherwise they
would never reach our noses. Fortunately most poison-
ous substances have a bad smell, especially the corrosive
ones. Still there are some poisonous gases and vapours
which have none, or only a very faint smell. They are
especially dangerous. We will learn about one of these
gases later on.
P. Then I'll take care.
M, We will go now to the nitrogen group. You know
a little about this already. It is not poisonous, because
we breathe it together with the oxygen in the air. But
in pure nitrogen, without any oxygen, animals must die,
I02 CONVERSATIONS ON CHEMISTRY.
because they require oxygen to live. You know some-
thing about phosphorus too.
P. Yes, it is in the heads of matches.
M. Right. From that you know one of its properties.
It catches fire very easily; even the heat resulting from
friction makes it do that. That is why it is used in
matches.
P. I saw in the dark lately, that the heads of matches
shone; there was a pale-green light, and the cook told
me it was because the matches had become damp. How
is that possible?
M. Phosphorus bums slowly if it is left in the air,
and in doing so shines as you saw. So that the small
quantity of phosphorus, which is contained in a match's
head, shall not burn of its own accord, the phosphorus is
mixed with gum, or hme, which dries, and forms a
covering that keeps oxygen out. In the damp this
covering is dissolved, and the phosphorus comes in
contact with air.
P. Yes; but when I wet a match in the room later,
it didn't shine.
M. That must have been a so-called Swedish match;
they have no phosphorus in their heads.
P. What does phosphorus itself look like?
M. Almost like wax. It is kept under water, because
it burns slowly away in air, as I said before. Since
it is very poisonous, it is better I should not give it to
you in your hand.
P. How is it made?
if. You think you could make it for yourself without
my permission! No, that isn't so easy. It is one of
the ingredients of bones, and is separated in a rather
comphcated way.
LIGHT METALS. 103
P. How can it be in bones if it is so poisonous?
M. Phosphorus as a free element is poisonous, but
its compounds are not. There you have another exam-
ple of how different elements and their compounds can be.
— ^Now we come to the last group. Besides carbon, which
you already know, you must learn about silicon.
P. Does silicon come from the Latin silex, flint?
M. Yes; flint consists of a compound of silicon and
oxygen; it is usually called silicic acid. Quartz, sand-
stone, rock crystals, and flint consist of it. Finally,
almost all rocks are compounds of sihca, so that the
element silicon is one of the substances that are found
in the greatest quantity on the earth's surface. — Now
that will do for to-day. I will only say that the elements
mentioned now go under the name of non-metals.
They form the larger division of the elements; the other
consists of metals.
P. I think I've learned a great, great deal to-day.
M. That was only a walk through our future work.
The real learning comes later.
14. LIGHT METALS.
P. How many different sorts of metals are there?
M. About sixty. As we do not know enough about
some, the number is rather uncertain.
P. But how can you find your way among such a
large number?
M. In the same way that you can find your way amongst
the much larger number of animals and plants: they are
divided into groups, in which those which resemble
each other are put together.
tC>4 CONVERSATIONS ON CHEMISTRY.
P. They do that with animals and plants according
to their shapes and organs; that can't be done with
metals.
M. That is not quite right; the crystals which form
when different elements are in their solid state show
some resemblance, like the shapes of plants and animals.
But metals have other properties which are remarkably
different among each other, while organized beings
resemble each other pretty closely; those are their
chemical properties or their capacity to form compounds
with other substances. Besides that, their physical
properties, lustre, colour, density, hardness, and so on,
are very different.
P. Then I must know the properties of all the elements
I am to learn about, if I am to understand and remember
their classification.
M. You need to know first of all only those which lead
up to, and complete, the classification. At present you
only need to know that the elements which I place in one
group possess definite resemblances in their properties.
P. Yes, that is true. What properties are the basis
of classification?
M. They are very different. It happens that the
groups which have been placed together because of one
definite property are almost always those which would
be made because of other properties. So the present
usual grouping is the result of quite a number of these
selections of properties. Those which, in each group, have
similar properties will be explained to you separately
later.
P. So there is a perfect order?
M. Almost, to the same extent, as there is order among
plants and animals. There, too, there are doubtful
LIGHT METALS. 105
points, either because the difference is too little or because
different methods of classifying lead to varying classi-
fication.
P. But it can't be that in such unchangeable things
as the properties of elements there can be contradictions ?
M, There are no contradictions in the properties,
but the irregularities of the somewhat arbitrary arrange-
ment that we have made —
P. Yes; then why isn't everything simply arranged
as in arithmetic or geometry ?
M. For this reason: we have only incomplete know-
ledge of the properties of the elements. Most of our
experiments, for example, are made at temperatures
which are not very different from that of a room, and
under ordinary atmospheric pressure. Our conceptions
of the properties of the elements would be quite different
if we knew how they were affected by all sorts of pressures
and temperatures.
P. Then the imperfection of the classification is only
due to the incompleteness of our knowledge?
M. That is quite possible, for experience has shown
up till now that a department of science becomes clearer
and more easily surveyed, the more exact and all-embrac-
ing our knovdedge becomes. But now we will go back
to our subject. We will divide metals into light metals
and heavy metals.
P. What is the meaning of light metals? All sub-
stances have weight, and so are heavy.
M. Quite right. Those metals whose density is less
than four times that of water are called light.
P. Why was four made the limit?
M. Because the other properties of metals are such,
that by making a limit here, it made their differences
106 CONVERSATIONS ON CHEMISTRY.
most clear. This is a case of the mutual help of dis-
tinguishing marks that I mentioned before. — Light
metals fall into three groups: the alkali metals, the
metals of the alkaline earths, and the metals of the earths.
These groups contain the following important elements:
Alkali Metals.
Metals of the Alkaline Earths.
Metals of the Earths.
Sodium
Magnesium
Aluminium
Potassium
Calcium
P. But those are very few.
M. They are by no means all. But I won't mention the
others just at present, because either they are so seldom
found, or have so little importance in their uses, that
you needn't bother yourself about them just now.
P. Is the aluminium which you have named the well-
known white metal?
M. Yes, and if you have had a piece of it in your
hand you will remember that it is extraordinarily light.
It is, in fact, only 2.7 times heavier than water.
P. Yes. Aluminium really is a light metaL But is
it true that it is made out of earth?
M. It is half true; only earth is not a definite chemical
substance, but an accidental mixture of all sorts of rocks
and their products from decay and time. But in nearly
all rocks and earth aluminium is found in the form of
a compound with oxygen. The different sorts of loam
and clay especially contain the element aluminium.
P. Ah, that is why it is called a metal of the earth.
But if it is so common, why is it so expensive?
M. It isn't so very expensive now; one pound costs
about twenty-five cents; that it is so much more expensive
than the substances it is obtained from is because it
requires a great deal of work to separate it from its
LIGHT METALS. 107
compounds. It was hardly known before the electric cur-
rent began to be used. The difference of price between
aluminium and its compounds, then, shows the greater
amount of work or energy which is contained in the
element aluminium, than in the compounds from which
it is prepared, and as you know work is never given as
a present.
P. Can you get the work out of the aluminium again?
M. Certainly. Here is a mixture of aluminium with
a compound of iron, iron oxide, which you already know.
If I light this mixture an immense amount of heat is
given off, the mixture glows white hot, the metal iron is
set free, and all sorts of welding and melting can be done
with the hot mass.
P. That is a pretty experiment. How was the mix-
ture made?
M. Aluminium powder and iron oxide are mixed in
the proportion of i to 3. Both substances must be thor-
oughly dried beforehand by heat. The lighting is done
with a small piece of magnesium ribbon (you will soon
learn about magnesium itself), which is made to burn
by means of a match. The mixture is placed in a
clay crucible, or in a cavitj, which you can make in a
dry brick.
P. What happens exactly?
M. Iron oxide, as you knew, is a compound of iron
and oxygen. If aluminium comes in contact with it
when hot, it unites with the oxygen, and the iron is
separated; as through the uniting of oxygen with alumin-
ium much more work is set free than is necessary to
separate oxygen from iron, a great amount is left over,
which appears as heat.
P. Is work the same thing as heat?
toS CONVERSATIONS ON CHEMISTRY,
M. In so far as the one can be changed into the other.
Vou can tell that work changes to heat because, by fric-
tion, heat is obtained. And to overcome friction, work
is necessary.
P. Yes, now I know. And a steam-engine makes
work from heat.
, M. Right. But now we must go back to our light
metals. Of the metals of the alkaline earths you prob-
ably already know magnesium.
P. Isn't it magnesium that burns so brightly?
M. Yes, magnesium is a light white metal, which can
be lighted, and burns with a bright flame. It is used when
a bright light is required and no electrical current is at
hand. For that purpose it is generally made in the form
of a narrow strip or ribbon. Here is such a piece of
magnesium ribbon that is brought in this form into
commerce. I light it, and you see how brightly it burns.
P. What is the white ash and the white smoke that
comes from it?
Af. That you ought to know for yourself. What is
combustion ?
P. A combination with oxygen. Then is the white
stuff an oxide of magnesium?
M. Yes. And the strong light is another example
that in this combination between oxygen and magnesium
there is a great deal of surplus work which shows itself
as light and as heat.
P. Then is light a sort of work?
M. Yes, certainly. You know that plants grow and
increase in light and form wood, leaves, and so on. The
wood you can burn and obtain heat from, as a sign that
there is work in it. This work has come from the Hght
of the sun, because plants can only grow in light.
LIGHT METALS, 109
P. Where is magnesium found?
M. Like aluminium it must be obtained from its
compounds by means of electric work. In nature,
compounds of magnesium, generally with oxygen, occur
in very large masses. Dolomite, which forms large
mountains, is rich in magnesium compounds; they also
occur in nearly all rocks.
P, What is magnesia that is used as medicine? Has
it anything to do with the metal magnesium?
M. Yes, it is magnesium oxide, the same substance
which is formed on burning the metal. The medicine,
Epsom salts, is also a compound of magnesium. All
these substances you will learn more about later on.
P. I should really like to have heard more about
magnesium: there are so many sorts of things connected
with it.
ilf . You will find the same thing with other metals.
Calcium, for example, as a metal, is very little known,
because it takes far more work than magnesium does
to separate it from its compounds, and it bums far more
easily.
P. Why should I learn about it now?
M. Because its compounds are extraordinarily exten-
sive; it belongs to the elements in which the earth's
surface .is richest. Limestone, of which large mountains
and countries consist, is one of its compounds; chalk
and marble are the same compound in rather different
forms.
P. But chalk, marble, and limestone are surely
different !
M, Yes, in their outward appearance. But if I take
a small piece of the three substances, and pour hydro-
chloric acid over them, they behave in the same way;
no CONyERSATIONS ON CHEMISTRY,
they froth up and let a gas escape. And the resulting
solutions give, in the same way, a white precipitate if
I add dilute sulphuric ^cid. And there are a great
number of other reactions which always occur which-
ever of the three minerals I use. Their difference is only
that dialk consists of far smaller particles than the other
two, and that limestone contains additional impurities,
which make its colour appear grey. But marble also
often contains impurities and appears red, sometimes
black. So the three are only different physically; chem-
ically they are the same.
P. Are there other compounds of calcium?
M. Innumerable. By pouring water on the burnt lime
which is got by strongly heating limestone, it heats
itself and swells up, and with more water forms milk
of lime, which, mixed with sand, is used as mortar.
Gypsum and cement are also compounds of calcium.
P. I'd like to learn more about them too.
M. Again, you must wait till later on for them; other-
wise we won't get through our talk. Now we have
still the first group — the alkali metals — to consider. Look,
in this glass there is sodium.
P. It looks white like silver. But why is the glass
sealed ?
M. Because sodium combines even at ordinary tem-
perature with the oxygen of the air. As no air can get
in through the glass the metal remains unchanged, and
its white colour and silver appearance can be recognized.
These grey pieces are also sodium.
P. But they look quite different!
M. That is only on the surface, where the compound
of oxygen has formed. If I cut off this layer with a
knife, the shining metal will be exposed.
LIGHT METALS, III
P. But it will soon be grey again!
M. Yes, it will combine with the oxygen in the air.
P. What sort of liquid is the piece of sodium in?
M. It is ordinary petroleum. I told you before that
it was made of hydrogen and carbon; it contains no
oxygen. That is why sodium can be kept in it, and is
protected from forming a compound with oxygen.
P. Then can sodium take oxygen out of a compound?
M, Certainly. I throw a piece of sodium into water.
It becomes hot, melts, and the ball dances about on the
water, always getting smaller. Now take care, a little
explosion will follow. See, now it is over, and all the
sodium has vanished.
P. Where has it gone?
M. It has united with the oxygen in the water, and
has become an oxide, which has dissolved in the water.
P. Is this oxide found in nature?
M. No, it can only be artificially made. But there
is another compound that is found in nature. Ordinary
salt is a compound of sodium.
P. With what?
M. With chlorine.
P. That can't be true, surely.
M. Why not?
P. Sodium is such an acrid stuff, and chlorine too,
and yet their compound makes common salt which we
can eat.
M. You have guessed wrong again, as if the ele-
ments were contained as such in their compounds. That
common salt is a compound of sodium and chlorine
tells you no more than that salt can be made with both,
and vice versa, both elements out of salt.
p. Is that really possible?
112 CONVERSATIONS ON CHEMISTRY.
M. You shall see it for yourself later on.
P. I can hardly wait to see and learn about all these
wonderful things.
M. At present we must speak about the last light metal
potassium. Here is a glass tube with potassium.
P. It looks just like sodium.
M. Yes, and behaves in a similar way. If I take
a piece out of the oil where it is kept and throw it
into water the effect is so strong that a violet flame is
the result.
P. Then potassium won't appear as a metal in nature ?
M. No! if there had been any to be found, it would
have seized all the water to be had, and changed into
a compound with oxygen.
P. What are the compounds of potassium ?
M. There are a great many. Among the substances
that you know I will name saltpetre. Further, potassium
is an ingredient of many minerals. Ordinary red felspar
contains potassium. There are potassium compounds in
the earth from rocks, and they are taken up by plants,
which need potassium to enable them to live. For that
reason potassium compounds are found on the ashes of
plants. They remain behind on burning, as they are
not volatile. They can be separated from the ashes
with water, and 'by evapoilating the water, are obtained
in solid form. The white, salty-looking substance which
is so obtained is called potash.
P. I would like to make that.
M, It is quite easy ; you only need to stir up wood-ash
with water and pour it through a filter (see page 15).
Then a clear liquid runs through, which tastes like soap,
and leaves a white or grey salt behind, if it is put in a
saucer in a warm oven. But take care that you use
HEAVY METALS. 1 13
only the ash of wood, and not that of coal, because that
doesn't contain potash.
P. I have learned so much to-day that I'm afraid I
shall never remember it all.
M, All that we have been speaking about will come
again later on when we learn the compounds of separate
elements. To-day I only showed you that you know
quite a lot of chemistry, that is to say, many substances
which you have noticed in daily life. You must certainly
gain first orderly knowledge of substances and their
behaviour, that is, real scientific knowledge.
P. I shan't fail for want of diligence and attention.
15. HEAVY METALS.
M. To-day we begin to talk about heavy metals.
Among these are the ones that have been longest known,
such as copper, gold, tin, lead, and iron.
P. Why were just these known first?
M. Gold is found as such in the earth. Copper, tin,
and lead are very easily separated from their ores, so
that it was possible to obtain them at an early epoch
without any great experience or skill. Iron came into
use much later, as it was more difficult to obtain. But
we will make a table again. And here also I will only
bring before you the most important metals:
Iron
Nickel
Copper
Silver
Gold
Manganese
Chromiutn
Leac
Tin
Platinum
Cobalt
Zinc
Mercury
P. I know almost all of these.
M. You won't know much about manganese. It is
a metal that is very like iron, and you have learned
114 CONIFERS AT IONS ON CHEMISTRY.
about its compound with oxygen in one of our earlier
experiments. It is the substance which we used when
preparing oxygen from potassium chlorate to make it
come off more easily.
P. Cobalt is blue; is it an element too?
M. No, the blue colour is that of a compound of the
element cobalt. Cobalt is also like iron, but keeps better
in air, and doesn't rust like iron. You know nickel?
P. Yes.
M. Some coins are made of nickel. Besides, cook-
ing-utensils are made of it. The metal is far whiter
than iron, almost like silver, and remains bright in
damp air without rusting. It is hard, and is difficult
to melt. For that reason it is a fairly valuable metal.
P. What happens to iron when it rusts?
M. It combines with the oxygen of the air and with
water. Therefore iron keeps much better in dry air
than in damp air.
P. What does nickel-plating mean?
M. It means covering over with nickel. With the help
of an electric current the metal can be deposited from
solutions of nickel compounds on to any sort of metal
object. As nickel keeps so well in air, these covered
or nickel-plated objects keep better than without this
covering.
P. I don't know chromium at all.
M. I won't tell you much about this element yet. It
is whiter than iron, very hard, and melts with great diffi-
culty. Many of its compounds are brightly coloured
and so are used as colours for pictures and painting.
But you know zinc ?
P. Is it the white or light-grey metal of which roof-
gutters and whole roofs and bath-tubs are made?
HE/tyy METALS. 115
M. Yes; it is much softer and more easily melted
than the other metals which were mentioned before. —
We now come to the copper group. You already know
that metal quite well.
P. Yes, and I know lead too ; it is so heavy.
M. Its density is*ii.4. It melts very easily, and is soft.
Most metals with low melting-points are soft.
P. And vice versa ?
M. No, it doesn't hold the reverse way. Gold and
silver are fairly soft, but have a high melting-point.
But it holds again for tin: tin is rather soft.
P. And it can be very easily melted. We did it on
New Year's day, and poured it into water. What made
the crinkled shapes that we got?
M. You should be able to answer that for yourself.
Tin melts at 235°. What will happen if you pour melted
tin into water?
P. The water will begin to boil. Now I understand
it: the water makes steam, and swells the Hquid
metal.
M. Right. And it hardens when it comes in contact
with the remaining water. — ^What do you know of mer-
cury?
P. That it is liquid at the ordinary temperature.
M. It is the only metal that has this property. It
isn't, however, the only liquid element, for bromine at
ordinary temperatures is also liquid. — You know silver
too?
P. Yes, from silver coins and teaspoons.
M. Mercury and silver are counted as precious metals,
and so are gold and platinum in the next group.
P. Why are they called that? Because they are so
expensive ?
Il6 CONVERSATIONS ON CHEMISTRY.
M' Not exactly for that reason, as there are other
much rarer elements, which are much more costly, and
yet are not called precious. No, they are called so
because they remain bright when heated, and don't
become black and ugly like other metals.
P. But why?
M. That you must answer for yourself. I have
already told you what happens to iron when it is heated
in air.
P. Yes, it combines with oxygen, and the other metals
will do the same. Can't the precious metals combine
with oxygen?
M. Certainly. Their oxides are also known. But
they have the property that when heated they decom-
pose into metal and oxygen. I showed it to you once
before with mercury.
P. Oh, so that is why their oxides can't be formed by
heating the metal, as they would at once decompose.
M. Right. It requires work to make these metals
combine with oxygen, and heat alone can't perform this
work.
P. Do the precious metals form no compounds?
M. Yes, some can be obtained if the precious metals
are treated with substances which yield work on com-
bination. Sulphur, for example, does so with silver and
mercury.
P. Can I see it?
M. Certainly. I put a drop of mercury in a mortar
and add some sulphur to it. Then I rub both together.
What do you see?
P. It is all becoming black. Now there is a fine
black powder, like soot. What is it?
M. It is a compound of sulphur and mercury. In
MORE ABOUT OXYGEN. li7
the same way silver can be combined with sulphur.
Rub some sulphur with a cork on a silver coin.
P. The silver is becoming brown and blackish grey.
M. There again is another combination of both
elements. Both metals unite directly in the same way
with chlorine, bromine, and iodine.
P. Aren't these precious, then?
M. No. But gold and platinum are still more precious,
as they don't combine with sulphur by rubbing them
together.
P. Don't they combine with anything?
M. Yes, they can combine with chlorine. But* this
compound decomposes into elements again on heating,
just as you saw with mercuric oxide. We will stop with
that for to-day.
P. Chemistry is a tremendously large subject.
16. MORE ABOUT OXYGEN.
M. To-day we will learn more about oxygen.
P. I know about it already.
M. Only ver)^ superficially, for you know only a very
small part of what is known about it. And what I am
going to tell you is only a little part of what is known.
P, But you know all about it?
M. No, I don't think there is a single man who really
knows all that is known about oxygen.
P. 1 don't understand that. If no one knows it, then it
isn't known.
M. One man knows one part, another man knows
another, so that the knowledge is present in somebody's
1 1 8 CONyERSA TIONS ON CHEMISTR Y,
mind, but not all in the same person's. Besides, almost
all is to be found in books, and is accessible for every one
who wishes. From time to time a man is found who
discovers as much as possible about it, and puts it all
together in a particular book, to save others the trouble of
searching. But he can only give extracts, and so one
who for some reason wishes to learn exactly what is
known of the subject must look over the books himself,
or by experiment arrive at the desired knowledge.
P. Is everything right that is found in books?
M. Most of it ; and when there is anything wrong, it
is no intentional error, but the author for some reason
made a mistake. A most remarkable and praiseworthy
thing in scientific literature is that almost every word
is written conscientiously.
P. But if someone has made an oversight and written
something wrong, the error would remain there forever.
M. Only until it is contradicted by some other fact
that is found. Then it is seen on which side the fault
lies, and after that one can generally find out how the
error came. But now we will go back to oxygen. You
remember how we made it before?
P. Yes, from a white salt. What is it called?
M. Potassium chlorate. It contains about two fifths
of its weight of oxygen, which it gives up when gently
heated, especially if a little oxide of iron or of manganese
be added.
P. You told me that already (page 114) but it strikes
me as so remarkable that I should like to see it. Can
you show me how iron oxide makes it easier for the
oxygen to come off?
M. Certainly. I am melting a Httle potassium chlorate
in a test-tube. What do you see?
MORE /iBOUT OXYGEN. 1 19
P. It melts; now it has become as clear as water;
now quite small bubbles are rising.
M, These are traces of oxygen. Now I take the
lamp away from the glass, and add a little oxide of iron
to it.
P. It froths like soda-water. Does the salt begin to
boil?
M. No, oxygen comes off suddenly. If I put- in a
burning splinter, it catches fire. You know that is the
test for oxygen. You see that even though the salt has
cooled a little on taking away the flame, the oxygen
comes off much more quickly on adding the oxide of
iron.
P. That is really very curious. Why does it happen?
M. The oxide of iron has acted like oil on a rusty
machine, or like a whip on a horse.
P. I don't understand that.
M. You are not the only one. It is a fact that many
chemical processes which go very slowly of themselves
can be accelerated by adding other substances to them,
even though the added substances undergo no permanent
change. The investigation of the question why these
accelerations, which are ascribed to catalytic action, really
take place is a difficult scientific problem, and perhaps
in a few years I may be able to give you an answer. In
the mean time we will use this fact as a convenient help.
P. When I know more I'll try to find out the reason
of catalytic actions.
M. That is a good plan. But now we will make some
oxygen. You know already how it is done. First I
will place this flask filled with water here, for we must
first expel the air from the flask by oxygen before I col-
lect it.
120 CONyERSATlONS ON CHEMISTRY.
P. But you will lose some oxygen in that way.
M. That doesn't matter ; if we want it pure, we must
make up our mind to that. You will always meet with
that same difficulty in future. Now I begin to heat,
and you see that soon bubbles rise out of the glass tube.
Now place the flask on the stand, but take care that you
always keep its mouth under water or else air will enter.
P. How quickly it's going!.
M. Yes; it will be better to take the flame away for
an instant. Now fill an empty flask with water and
have it ready.
P. But how can I turn it upside down without letting
the water run out?
M. Hold your thumb on the mouth.
P. My thumb is too small.
M. Then take your hand or a piece of cardboard, or
anything flat. The best thing is a cork that fits it.
P. Now the first flask is full of oxygen.
M. I close it under water with a cork, and can take
it out and put it aside.
P. Why do you put it upside down?
M. Because generally the cork doesn't fit tight, and
the water then prevents the oxygen from coming out.
Now the second flask is nearly full; get another flask
ready.
P. I didn't think that so much oxygen could come out
of so small a quantity of salt ; the sixth large flask is half
full, but it has stopped coming now.
M. Yes. Now we will take the tube out of the water;
if we didn't, the water would rise up into the flask and
break the hot glass.
P. What a lot of things there are to think about!
M. Yes, the art of experimenting is not to require
MORE ABOUT OXYGEN. 121
to think about such things, but to do them involuntarily.
Now we will do what we had to put off before; we will
calculate the density of oxygen.
P. Calculate? But we must first measure it.
M. The measurements are already made. I used lo
grams of potassium chlorate; it contains about 4 grams
of oxygen; more correctly 3.9. Our flasks are each
half a litre or 500 cubic centimetres in capacity, as you can
see by the 500-mark which is stamped upon the bottom
of each. We have thus collected somewhat less than
3 litres of oxygen, so each litre weighs in round numbers
1.3 grams, and each cubic centimetre 0.0013 gram, so
(see page 48) the density of oxygen is equal to 0.0013.
P. I shouldn't have thought the calculation could have
been made so easily.
M. It was easily made, but it was not exact. I wanted
to show you how to arrive at a knowledge of such values.
It wasn't my intention to make an accurate measure-
ment.
P. One thing more. You said that the weight of the
oxygen from 10 grams of potassium chlorate was 3.9
grams, but not how you found it out.
M. That's not difficult. You weigh the test-tube with
the chlorate before heating, and then afterwards.
P. I see it now. The loss of weight is equal to the
weight of the oxygen that has come off.
M. Yes. Here you have an application of the law of
the conservation of weight.
P. So I have used a law of nature without knowing
it. What is the use of stating these laws of nature when
you use them without knowing them?
M. It was an accident that you used it rightly. It
is just as easy to make a wrong use of them, and in order
122 CONVERSATIONS ON CHEMISTRY.
to avoid that the law must be expressed and used inten-
tionally. This is troublesome at first, but later on, if my
teaching makes the right impression on you, whenever
you learn anything new, you will find it necessary to
state it as a law of nature.
P. I don't think I shall ever get as far as that.
M. We mustn't forget that we are speaking of oxygen
all the time. When we collected it over water, did you
notice anything strange?
P. I don't think I did.
M. The bubbles of oxygen rose through the water
without diminishing in size. That is a proof that
oxygen is insoluble, or very sparingly soluble in
water.
P. Can gases dissolve in water, then?
M. Certainly. You have an example in soda-water.
As long as it is in the bottle it looks quite clear, but when
you pour it out a quantity of gas escapes which was
dissolved before.
P. Yes, I have seen that. But why does the gas escape
when you pour it out?
M. Gases dissolve iri water and other liquids more
readily at high than at low pressures. In the bottle the
solution is at pretty strong pressure, and when the bottle
is opened the pressure is reheved, so that the gas escapes.
P. Ah, that is the reason why it pops and foams.
What sort of gas is it?
M. It is carbonic acid gas, the same gas which is
produced when carbon burns in air or oxygen. We
shall get to know more about it afterwards.
P. Then we ought to be able to make carbonic acid
gas out of smoke.
M. That doesn't work; for in smoke the gas is mixed
MORE ABOUT OXYGEN. 123
with much nitrogen of the air, and besides, it contains
disagreeably-smelling stuff from the coal.
P. I only meant it as a joke.
M. But the proposal is quite a sensible one. If
carbonic acid gas were an expensive gas, it would be
worth considering whether it couldn't be separated
from the mixture and purified. But because such a
separation costs trouble and money, the question is
asked, can it not be made in a cheaper way? The
answer to that question is the foundation of the chief
part of chemical industry. But we will go back to
oxygen. It is very sparingly soluble in water; while
water dissolves its own volume of carbonic acid gas,
it dissolves only about a fiftieth of its own volume of
oxygen.
P. But if it is more strongly compressed?
M. It remains the same. If a gas is compressed more
strongly, more goes into the same volume, and water
dissolves exactly as much more. On the other hand,
the proportion varies with the temperature; the higher
the temperature the less gas dissolves. What do you
notice when fresh spring-water is allowed to stand in a
room for some time?
P. Do you mean the little bubbles of air that stick
to the side of the glass ?
M. Yes, that is what I mean. When the cold water
which is saturated with gas warms up, part of it must
escape, and it does so in the form of bubbles, which
gradually grow larger, and finally separate and rise.
We have learnt something about the behaviour of oxygen
when it is kept in a flask by itself, and when it is brought
together with other bodies. Now we will get to know
about it in the free state.
124 CONyERSATIONS ON CHEMISTRY.
P. I am curious to hear about that.
M. You know that it is a constituent of air, and indeed
the most active. The other constituent is called nitrogen,
and animal life cannot survive in it, nor can flames burn
in it, but go out. As air penetrates everyv^here, so oxy-
gen can penetrate everywhere, and it combines with sub-
stances which are present; that has happened as long as
our earth has been in the same condition as it is now;
that is, for thousands and thousands of years. The
consequence is, that everywhere on the earth's surface
compounds of oxygen are to be found. Most of the
substances which we know contain oxygen. Compounds
of other elements with oxygen are called oxides. The
word oxygen comes from the Greek and means an acid
substance.
P. What has it to do with acid ? It isn't acid, surely ?
M. It occurs in many acid substances. It was formerly
believed that its presence was necessary to the existence
of acid substances, but that has subsequently been found
to be erroneous.
P. Why did they keep the false name?
M, Nobody thinks about it now, so it doesn't matter.
But we will leave the name and go back to the thing.
You know that by burning fuel we not only warm our
houses in winter, but we drive our machines, lift weights,
in fact, do all sorts of work that we require to do. Burn-
ing means union with oxygen. How does it happen that
that enables us to do work?
P, Oh, I know that from our former lesson. Burn-
ing is a chemical process by which energy is set free.
M. I am glad you remembered it. Now I will give
you a riddle. How does it happen that the coal doesn't
bum in our cellar?
MORE ABOUT OXYGEN, 125
P. Because there is nothing there to set it on fire.
M. How can things be set on fire ?
P. You put other burning stuff beside the coal till it
begins to burn.
M. That is not a sufficient answer. What happens
to the coal when you put burning stuffs beside it?
P. Now I've got it. The coal gets warm, and so catches
fire.
M. That is right. So hot coal can unite with oxygen
and cold coal can't. And that is the reason why
coal burns in the fire, and not in the cellar. But now
I will tell you something. It happens not infrequently
that coal which is left lying in large heaps catches fire
of its own accord and burns without anybody's having
lighted it. Such a heap gets hotter and hotter, and
when it is not cooled by spreading it out, it begins to burn.
P. I can't understand why. Where does the heat come
from?
M. That is a sensible question. It comes from the
burning of the coal.
P. But that only occurs later on.
M. No, the coal is always burning. Only this hap-
pens so slowly at low temperatures that the tempera-
ture rises very slightly, and so you can neither see it
smoke nor catch fire. When large heaps of coal, how-
ever, lie together, so as to prevent heat from escaping,
the temperature rises; then the burning takes place more
quickly and the temperature rises higher, and finally rises
so high that the coal begins to glow and bursts into flame.
P. I can't imagine coal actually burning in the cellar.
M. I will remind you of something else. Do you
remember what became of that log that was lying out
in the yard?
126 CONVERSATIONS ON CHEMISTRY.
P. It is just the same as it was.
M. No, that is not exactly true. If wood lies for a long
time it decays. Do you know what that means ?
P. The wood gets rotten and Hght.
M. Yes, and it gets smaller and smaller and finally
disappears.
P. What has become of it?
M. It has been burnt too. When oxygen is kept away
from wood, it doesn't alter Hke that.
P. But how can you .call it burning when you can't
see a flame?
M. Burning in the chemical sense of the word is
combination with oxygen, whether a flame is visible or
not. For whether a flame or glowing appears depends
upon the temperature rising high enough, at least to 500°;
below that substances don't glow, because they send out
no light. Whether the temperature rises so high doesn't
depend upon the chemical change, but on whether the
heat is sufficiently kept in.
P. Are there many combustions without light and
heat?
M, Plenty. But without evolution of heat, such
"flameless combustions" don't take place. Just as
much heat is evolved as if combustion had taken place
with a flame. When a chemical change takes place,
the amount of heat evolved depends on the beginning
and the end; it doesn't matter if it takes a long or a
short time.
P. But when the coal on the fire burns brightly, surely
it gets hotter?
M. The amount of heat which a definite quantity of
coal gives out is always the same. But of course if you
burn more coal in the same time, the fire will be hotter.
MORE ABOUT OXYGEN. 127
P. I really don't quite understand that.
M. The fire gains heat by the burning of the coal
on the one hand, but on the other hand it loses it by
heating the room. It is something like pouring water
into a bucket with a hole at the bottom. The quicker
the water runs in the higher it will stand in the bucket.
But that has nothing to do with the total quantity of
water that you pour into the bucket.
P. Yes, now I understand. When a tree decays, it is
like water running so slowly into the bucket that it's
never visible. But how can it be found out that as much
heat escapes as when ordinary burning takes place?
M. That is deduced from the law, that energy neither
disappears nor is created. That has been proved and
confirmed in innumerable cases, and it can be taken for
granted in cases where it has not yet been proved.
P. But it's surely possible that it may prove to be false
in some one case.
M. Certainly. But then other things would show it
was wrong, and the error would soon be discovered.
What do you know about the relations of animals to air?
P. They can't live without air, so I always make holes
in the lid of the box in which I keep my silkworms.
M. But there is air in the box anyhow, along with
the silkworms, so what is the use of the hole?
P. But the animals require fresh air.
M. Why?
P. I was taught that. People require fresh air if they
are to keep healthy.
M, Quite right. The important point is that both ani-
mals and men shall get enough oxygen. Breathing con-
sists in pumping oxygen into the lungs, where it is taken
up by the blood and led through all parts of the body.
128 CONVERSATIONS ON CHEMISTRY.
P. What's the good of that?
M. To burn the body.
P. You're surely in fun?
M. No, I'm really in earnest. The process of the
body is exactly the same as with the coal in the cellar
and the decaying wood. Certain substances in the body
combine with oxygen, although not so quickly as with
burning wood.
P. Is that what makes the body warm?
M. Certainly. A dead man no longer breathes, so
his body gets cold. But that is not the only effect pro-
duced. The body does all sorts of work, which must
be produced from something, because work can't be
made of nothing. This work or energy is produced
from its combustion.
P. Then surely both our bodies ought to have burnt
up long ago?
M. Quite right. If we were not always introducing
new combustible matter. That happens when we take
in food.
P. Then I ought to be able to eat wood and coal.
M. Yes, if you could only digest them; that is, if your
stomach was able to change them into soluble com-
pounds, which would be carried with the juices of the
body to all the parts, where they could combine with
oxygen. For that matter, cows can digest wood if it
is given them sufficiently fine. The substances of which
grass and hay are made are not very different from wood.
P. Does the food burn in the lungs?
M. You mean because air enters the lungs in breath-
ing? No, oxygen of the air is taken up by the blood in
the lungs and passes through the arteries into . all the
tissues of the body; and there it meets the dissolved
HYDROGEN. 129
foods and bums them up. Besides, food has another
use: it replaces the used-up parts of the body. If you
think of your body as a steam-engine, food is not merely
the coal which makes it go, but also the metal with
which it is repaired.
P. Is that the case with all animals, or only with
warm-blooded ones?
M. You think that cold-blooded animals don't require
it because they are not warm? That isn't right, because
they are all a httle warmer than their surroundings,
and they all breathe. Ail animals require food and
oxygen because, besides keeping themselves warm, they
have to do work. They move about.
P. But plants don't move. What about them?
M. With plants there is a different state of affairs
which you can't quite understand yet. We will come
back to them, and then you will see these things in a
connected manner.
P. It has been a jolly lesson to-day.
17. HYDROGEN.
M. We will talk about hydrogen to-day. What do
you know about it?
P. It comes in water.
M. That is not expressed well; because it can be
obtained from water, hydrogen is an ingredient of water.
What other ingredient has water besides that?
P. I think you said oxygen.
M. Right. Water consists of hydrogen and oxygen;
that is to say, water can be formed from both these
elements, and in the same way both these elements can
t^O CONyERSATIONS ON CHEMISTRY.
be obtained from water. How do you think oxygen
could be made from water?
P. I don't quite know. Perhaps water could be
heated and it might decompose into the two elements,
just as oxide of mercury decomposes into its ingredients.
M. That is quite a good suggestion. But you already
know what comes when water is heated.
P. Yes, steam.
M. Right. Steam is only water in another form.
P. Perhaps it requires greater heat.
M. You have hit upon the right thing; if steam is
very strongly heated, it really decomposes into oxygen
and hydrogen. But when the mixture is cooled, it com-
bines again *'to form" water, and one can only tell by
a special artifice that it has been decomposed. Besides,
only a mixture of oxygen and hydrogen would be obtained,
and as both elements are gases, it would not be easy to
separate such a mixture.
P. Then a way must be found out to hold fast the oxy-
gen somehow. Can't it be made Hquid, like the mercury
in the decomposition of mercuric oxide?
M. Yes: to do that the gas mixture must be cooled
below — 1 80° C. That is too inconvenient a way. I
will show you another: we do not separate the oxygen
alone, but as a compound with some other element,
and arrange it so that the compound is not volatile.
P. I don't quite understand.
M, I will tell you. We pass steam over glowing
iron. You know that iron combines easily with oxy-
gen.
P. Yes, it burns with a lovely rain of sparks.
M. Now the iron acts on the steam in such a way
that it takes the oxygen and combines to form iron
HYDROGEN, ^31
oxide; the hydrogen then remains over. Iron oxide is
a soHd substance even at a red heat, and therefore
remains where the iron was; but hydrogen is a gas,
and passes further along; it can then be collected over
water in the same way as oxygen.
P. That still seems very strange to me.
M. I will give you a simile. Oxygen is a bone which
the cat hydrogen had to start with. Then the dog iron
comes along and takes the bone from the cat, and the
cat hydrogen must run away without the bone.
P. Then iron is stronger than hydrogen, and so takes
the oxygen away.
M. The old chemists made the thing out to be some-
thing like that, and for the present you may rest content
with that simile. Later on, when you know more about
chemistry, you shall have more definite examples.
P. May I see the experiment ?
M. It is not quite simple to arrange, for a fairly
strong heat is required. The best way is to fill a piece
of hard glass tubing with a bundle of iron gauze, to
heat the middle till it glows, and lead the steam over it
from a flask in which water is boiling at the other end
of the tube; a glass tube is attached, which is allowed
to discharge into an inverted flask under water. Then,
exactly the same as with oxygen, the gas-bubbles rise and
collect in the flask.
P. What a shame I can't see it!
M. I will show you another experiment instead, with
which you can see much the same kind of thing. You
remember that salt contains a metallic element which
is called sodium. Here is some of this metal. I have
already shown you (page in) that it combines rapidly
with oxygen, and that it can also take it out of water. —
132
CONVERSATIONS ON CHEMISTRY,
Now I take a little piece of sodium the size of a pea,
wrap it up in a piece of filter-paper, and stick it with a
pair of tongs under the inverted tube, which stands in
the water (Fig. 23).
Fig 23.
P. The sodium is slipping out of the paper! Now it
seems to be boiling, and some air has collected in the
tube.
M. Sodium acts in the same way as I told you iron
did, only at ordinary temperature and much more
quickly. It takes the oxygen of the water and sets free
tlie hydrogen.
P. But why did you wrap it up in paper?
HYDROGEN. I33
M. Without doing that it would have been difficult to
put it under the tube, as it would have sprung out of the
tongs. It gets heated and melts. As we haven't ob-
tained a great deal of hydrogen, I will repeat the experi-
ment and you can see that sodium glides about on the
top of the water like a liquid ball.
P. Why didn't you take more sodium at once?
M. Because the experiment is not quite without danger
if large quantities are taken. There are often impurities
in the sodium which make it explode, so that only small
quantities must be taken in order that an explosion may
not be dangerous. Remember this when you make the
experiment alone.
P. Yes, but tell me what has become of the compound
of sodium and oxygen which must have been formed?
M. A very good question! Well, as it is neither on
the top of the water nor under the water, where can it
be?
P. In the water? But the water is still quite clear.
M. Quite right. So what properties must the com-
pound have? Think of our first talks about sugar and
copper sulphate.
P. I know! It has dissolved.
M. Quite right. Taste the water so as to convince
yourself.
P. Horrid, like soap!
M, You have discovered one reaction of the compound
which is formed. But we will speak of that later. Let
us pay attention to hydrogen at present. What does it
look like?
P. Like air.
M, Yes, hydrogen is a colourless gas. Now I take
the tube out of the water, closing the mouth with my
134 CONVERSATIONS ON CHEMISTRY.
thumb, and take away my thumb when I bring it near a
flame. What do you see?
P. The hydrogen appears to burn; but the flame is
very pale.
M. Quite right. Hydrogen is a combustible gas.
But in order to learn more about its properties we should
have to put sodium again under the mouth of the tube,
and that would be tiresome. I will rather show you
another method of making hydrogen, by which it is
much easier to produce large quantities. For this pur-
pose we take other compounds of hydrogen which give
it up more readily than water does. Such a compound
is hydrochloric acid; as its name implies it consists of
hydrogen and chlorine.
P. Is that the same chlorine that is contained in com-
mon salt?
M. Certainly; there is only one kind of chlorine.
Here is a solution of hydrochloric acid in water, as it is
sold by the druggists.
P. It looks just like water.
M. Yet it is not water. I pour some drops into a
wine-glass, and fill it half full of water; taste it.
P. Will it have as bad a taste as the last ?
M. No, quite different.
P. Yes, it tastes sour. But not very pleasant, and it
makes my teeth rough.
M. Yes, because it tastes acid it is called an acid.
P. Why did you pour in so much water?
M. Because strong hydrochloric acid is poisonous,
though dilute acid isn't. The reason your teeth felt rough
was that the acid attacks the substance of which the
teeth are made. But now we will begin our experiment.
I have here, in a flask, clippings of sheet zinc. Now we 11
HYDROGEN.
135
put into the flask a cork provided with two holes.
Through one passes a tube with a funnel at the top,
and it reaches to the bottom of the flask; and through
the other a short bent glass tube to which I connect my
delivery-tube with a piece of rubber tubing — the one I
used before for oxygen (Fig. 24). Now I pour hydro-
FlG. 24.
chloric acid through the funnel, and at once you see
gas coming off.
P. Quick! Place the flask over it to catch it.
M. No, I will first collect some gas in a test-tube.
Stop, here is the first one full. I Kft it out and hold it
to the flame. What happens?
P. Nothing. It must have been the air that was in
the flask.
M. Quite right. Now I repeat the experiment.
P. That gave a loud crack.
M. I will collect some more samples. You see, the
136 COhiyERS/iTIONS ON CHEMISTRY.
first ones explode, but now the gas bums quite quietly,
like the hydrogen we made by using sodium. Now we
can collect it in flasks, and when the evolution of gas
slackens, we only need to add a little more hydrochloric
acid and it begins again.
P. Please explain all this to me.
M. With pleasure; First the production of hydrogen
from hydrochloric acid and zinc; that is just like the
formation of hydrogen from water and iron. The
chlorine takes the zinc rather than stay with the hydrogen,
and so the hydrogen is set free. It is very convenient
that this takes place at the ordinary temperature, and with-
out the necessity of using a dangerous metal like sodium.
P. I understand that. But what niade it pop ?
M. Look, here I have a test-tube that is only half
filled with water. I close it with my thumb and place
its mouth under water. Now the tube is half full of air.
I displace the water from the other half of the tube with
hydrogen, which is not explosive. If I bring this tube
with a mixture of hydrogen and air near the flame —
P. By jove! what a thundering crack!
M. You see that a mixture of air and hydrogen ex-
plodes, though pure hydrogen doesn't. If I were to
light such a mixture in a flask, it would burst, and the
pieces might cause serious injury. Now, as there was
air originally in the bottle, it would have made a dangerous
mixture; and it was only after the air had been driven
out by hydrogen that pure hydrogen escaped. Remember
that you must always test the gas when you make hydrogen
in this way, and not collect the gas before it burns quietly.
P. So the explosion is a test for air in the hydrogen?
But why did it pop?
M. Because the hydrogen was completely mixed with
HYDROGEN. 137
the oxygen, which it required for burning, and the flame,
when it once starts, spreads immediately through the
whole mass. But when pure hydrogen burns in the
air, combination can take place only when the two
gases touch. The shape of the surface when this
takes place is the same as that of the flame. Can you
tell me why a quietly burning flame like that of a candle
has a conical shape?
P. Let me think. Yes, the burning gas rises and
burns, and because it grows less the flame gets narrower.
M. Quite right. Now let us go back to hydrogen.
I fill two tubes with it, and leave one with its mouth up
and the other with its mouth down. Which will the
hydrogen stay in?
P. When you ask questions, I am afraid of some catch,
and am likely to answer wrongly. So I'll say the oppo-
site of what I think. The hydrogen will stay in the tube
with the mouth downwards.
M. Let us try. First I bring into the flame the tube
which has its mouth upwards, and try to set its contents
on fire; nothing happens, and when I hold a burning match
in it, it goes on burning; and so the tube contains air.
Now the other tube. I hold its mouth above the flame —
P. I was right after all. The hydrogen stayed in
it. It burns with a pop. That is most astonishing.
M. Now think. What did I tell you about the density
of hydrogen?
P. That it was the lightest of all substances. But
still it has some weight and ought to fall. Oh, now I
know. It is lighter than air, and so it floats up in the
air like a cork in water. But in an empty space it ought
to fall.
M. So it would if it were a solid or liquid. But a
1 3^ CON VERSA TIONS ON CHEMIS TR Y.
gas spreads all through an empty space till it fills it
equally throughout. Now do you understand the experi-
ment?
P. Yes; the hydrogen tries to rise in the air, and if it
can find an opening above, it escapes ; but if the opening
is below it, it must stay there.
M. Quite right. Now you deserve a treat, and I
will show you a pretty experiment which will illustrate
its behaviour even better. I have made some soap-suds.
Now, by means of a piece of rubber, I pin to the gas
dehvery-tube a piece of glass tubing stopped loosely with
cotton-wool, and plunge the end below the soap-suds.
P. You can really blow soap-bubbles with hydrogen?
M. Yes, and here is a very big one; it separates from
the soap-suds, and rises like a balloon.
P. Oh, how jolly! But what is the use of the cotton-
wool in the tube ?
M. The hydrogen carries innumerable little drops
of acid with it as a sort of mist, and when these
touch the soap-bubble, it bursts. But the little drops
stick in the cotton-wool and don't get into the
bubble.
P. Are the big balloons that are sold in shops filled
with hydrogen?
M. Yes.
P. I used to have one, and the first day it went up
all right, on the second it would hardly rise, and the
third day it wouldn't rise at all. Did the hydrogen
grow heavier?
M. No, but hydrogen is such a fine-grained stuff that
it can't be kept in by a thin sheet of india-rubber; it
passes out, and some air enters instead.
P, Oh yes, I remember my balloon got much smaller.
OXYGEU AND HYDROGEN. 139
I thought at first that the mouth hadn't been tightly
enough tied, but it was.
M. Quite right. You see you shouldn't keep hydrogen
in any kind of vessel for a very long time; it generally
gets out and air enters, making an explosive mixture.
18. OXYGEN AND HYDROGEN.
M, What did you learn yesterday about hydrogen?
P. That it can be made from its compounds by taking
away by means of another substance what it is com-
bined with. It can be set free from water, in which it
is combined with oxygen, by iron or sodium.
M. And how do the two metals dififer in doing it?
P. Iron does it only when glowing, sodium does it at
the ordinary temperature.
M. And further?
P. You can take hydrochloric acid and zinc. The
zinc takes the chlorine, and the hydrogen comes out.
M. What properties has hydrogen?
P. It looks colourless, like air, but it weighs much less.
But you never told me how much hghter it was than air.
M. Its density is about 14 times less than that of air.
One litre of hydrogen, like what we have in the flask,
weighs less than Yn gram. What more do you know
about hydrogen?
P. It burns in air, and if it is first mixed with air, it
gives a loud bang, because the whole mass burns suddenly.
M. Quite right. What is made from hydrogen when
it burns?
P. You never told me.
M. You ought to have been able to discover it for
yourself. Just think a minute. What happens on burning ?
140
CONVERSATIONS ON CHEMISTRY.
P. The substances combine with the oxygen of the air.
M. Right. Now if hydrogen combines with oxygen,
what is made? Don't you remember that we have just
been speaking about such a compound; which was it?
P. You told me about water. Should water be made ?
M. Certainly, water is made. We soon see it. Don't
you remember how I showed you the formation of water
with a burning candle?
P. Yes, with a large beaker that was held over it.
It became covered with drops of water.
M. We can do that, too, with a hydrogen flame. I
fasten, to the apparatus, a glass tube the end of which
I have made narrower, and let the hydrogen burn from
it (Fig. 25). There, you see the drops at once.
Fig. 25.
P. How do you make a point like that?
M. You hold the tube in the flame, turning it till
the place is quite soft, then you pull it apart length-
ways, and cut through the narrow part with the glass-
cutter.
OXYGEN AND HYDROGEN.
141
P. Please let me do it. Now the tube is soft, and
now I pull it. Oh, it is as thin as a hair!
M. You pulled too hard and too quickly. Besides,
this thin hair is also a tube, as glass doesn't fall together
with pulhng.
P. Really? I can hardly believe that there can
be such a thin tube.
M, Break a piece o£f and put the end in the ink and
Fig. 26.
then you can see how the black liquid comes through.
But we must return to our hydrogen. Hydrogen can
combine not only with free oxygen, but can take oxygen
from other compounds. Do you remember mercuric
oxide? What sort of substance was that?
P, A red powder; a compound of mercury and oxygen.
M, Yes. I take a little mercuric oxide, put it in a
1 4 2 CONyERSA TIONS ON CHE MIS TR Y.
glass tube which I attach to the hydrogen apparatus,
let the hydrogen pass over it, and heat it carefully (Fig. 26).
P. Mercury separates again.
M. Right, but further?
P. There are clear drops that look like water; is it
water?
M. Yes. This time the hydrogen has taken away the
oxygen from the mercuric oxide to form water, and the
mercury is set free.
P. Does that happen with all oxygen compounds?
M. Not with all, but with a great many. Most
oxides of the heavy metals can be thus changed into
metals. This change is called reduction, the opposite
of oxidation. The changing of a metal into its oxide
is called oxidation, the changing of an oxide to the metal,
a reduction. As hydrogen makes this change possible
it is called a reducing agent. Notice this name.
P. I have learned a great deal that I didn't know
before.
M. I will make it easier for you by showing you some
more experiments. This black powder is called copper
oxide. It is easily formed if copper is heated for some
time in the air. I put some of it in a tube, pass hydrogen
over it, and heat it again; do you see what the copper
looks like?
P. Yes, the powder is getting red like copper, and
again drops of water are falling in the tube.
M. I take away the flame and let it get cold, while the
hydrogen is going through. Now I can shake out
the red grains, and if I rub them in the mortar you will
see they will shine like metal.
P. How pretty! Why do they only shine after they
are rubbed?
M. The copper was not even and smooth before.
OXYGEN AND HYDROGEN. 143
As the oxygen has separated from the copper oxide,
the copper remains behind Hke a sponge. — ^This yellow
powder is called oxide of lead and is a compound — •
P. Of lead and oxygen.
M. That is a good answer. I'll allow you to reduce
it yourself for that. Do it in the same way as before.
P. Bright drops like mercury have appeared; is that
lead?
M. Yes; as lead melts very easily, it is obtained at
once in a liquid form. Pour these drops onto a piece of
paper and then you can see how they solidify into a
soft and unelastic metal which is easily bent. Those
are the properties of lead. But now we are going to
do a special experiment. This is the iron oxide which
we obtained before by burning iron powder in the air.
We are going to reduce this by means of hydrogen.
P. How can that happen? You told me yourself
yesterday that iron is stronger than hydrogen, because
it takes the oxygen out of water and drives away hy-
drogen. So how can hydrogen become stronger than
iron?
M. One must make experiments even when one
thinks they won't come to anything. For every con-
clusion we draw may be erroneous, and must be tested
by experiment.
P. I am really curious about it. Do you see, nothing
is happening; the broken bits only become a little blacker.
M. Just notice carefully the further part of the tube.
P. H'm! There really appear to be drops of water
coming there. On the one hand, it looks as if nothing
were happening, and on the other hand, as if something
were happening after all.
M. I will let it cool again while the hydrogen is still
144 CONVERSATIONS ON CHEMISTRY.
passing over it. Now just rub the black mass in the
mortar, as we did with copper.
P. It is becoming bright too
M. Then it is metaUic iron.
P. Now please tell me how it is possible that there
can be such a contradiction. I thought that laws of
nature always held.
M. What law of nature has changed here ?
P. One force cannot well be greater and smaller
than the other. First iron was stronger than hydrogen,
and afterwards hydrogen was stronger than iron. That
is surely a contradiction.
M. The contradiction only lies in this, that you look
upon the reason of chemical change as a mechanical
force; a force Hke this doesn't let itself be known or
measured beforehand.
P. What is it, then?
M. If I were to answer this question, you wouldn't
understand me. You must know many facts about
chemistry before you can think of connecting them by a
theory.
P. But can't you say something that would put me on
the right track?
M. Yes; out of your own wrong example; one man
can carry a certain amount of water; but if much more
water comes it will carry the man away.
P. So you mean that in chemical changes it depends
on which substance is present in the greatest quantity.
M. Something like that. But we must go back to our
hydrogen. You know now that in the combining of
hydrogen with oxygen water is formed, and that for
this purpose oxygen can be taken out of other compounds.
But there is still something else that happens: I set my
OXYGEN AND HYDROGEN, 145
hydrogen apparatus going again, and after the mixed
gas has gone, light the hydrogen. You see that the
flame is fairly pale.
P. At first it is always bluish, but afterwards it
becomes lighter, and looks yellow.
M. That is because the glass tube from which the
hydrogen bums becomes hot. The element sodium
is contained in glass, as you already know. A Httle
evaporates from the hot glass, and this vapour colours the
flame yellow.
P. How is that?
M. Glowing sodium sends out a yellow light, just as,
for example, the metal copper reflects red light. The
yellow colouring of the flame is a test for sodium; it is
always to be found when sodium is present, and is absent
when there is none there.
P. But nearly all flames are yellow.
M. In nearly all burning materials sodium is present,
and a very little is enough to make the yellow colour.
But we can make a pure-coloured hydrogen flame. I
have here a little piece of
platinum-foil; I make it
soft by heating, and then
wrap it firmly round a
knitting-needle; so I get
a very serviceable little A
tube of platinum. I put ■ ^
a few millimetres of this in
a glass tube which is slightly wider than it, and heat the
place. You see how the glass tube lies round the plati-
num ? Now it is melted and closed up all round, and I
have a burner-tube with a platinum tip which I can
afterwards bend to a right angle. (Fig. 27.)
r'
U6 CONVERSATIONS ON CHEMISTRY.
P. Why platinum?
M. Because this metal is only melted with great diffi-
culty and is not easily attacked. When I attach the
tube to the hydrogen apparatus, I can leave the gas
burning for hours, and the flame will never become
yellow. Now I hold a morsel of platinum wire in the
hydrogen flame. What do you see?
P. The wire shines very brightly; the flame seems
to be very hot, then.
M. Quite right. A glowing body glows more brightly
the hotter it is. With gases this is not the case: glowing
hydrogen vapour gives very little light; that is why the
hydrogen flame is so faint, while it makes every solid
body that it reaches glow so brightly.
P. Every one?
M. Every one that doesn't melt or turn to vapour.
Here I have a fragment of incandescent mantle. Just
look how brightly it glows. And iron wire begins to
glow brightly at first too, but it soon melts and burns.
Very well, then, tell me what is formed in the flame
besides water?
P. Heat.
M. Right. What is heat ? Remember what we spoke
about a short time ago, when we were talking about
combustion.
P. Yes, you had a special name for it : I think energy.
M. Quite right. What is energy?
P. Everything that comes from work and can be
changed into work. How can you get work from burn-
ing hydrogen?
M. Now you heard for yourself what a loud bang a
mixture of hydrogen and air made, and I told you also
that it could break glass. Work is used up for that.
OXYGEN AND HYDROGEN. 147
P. A funny sort of work. Mother would very soon
put a stop to it if I wanted to break her glasses and said
I was at work.
M. It is work all the same, as it requires a certain
amount of exertion. To be sure, it is not useful work.
But when the miller grinds his corn, his mill does similar
work, and that is useful.
P. Can any useful work be done with the explosive
gas?
M. Certainly. There is a certain sort of machine
in which an explosive mixture of air and coal-gas is
burnt. The explosion drives a piston forward, and as the
machine turns further, gas and air are again sucked in
to form the explosive mixture, and this is again exploded,
so that the piston each time receives a powerful push.
Such gas-engines are now made of the largest dimen-
sions, and in many respects are much better than steam-
engines.
P. Are engines of motors made like that? They puff
in the same way.
M. They are something like, only with them the explo-
sive gas is made with benzine vapour.
P. Then explosive gas can be made with all sorts of
things ?
M. li di combustible gas or vapour is mixed with as
much air or oxygen as is necessary to burn them up, an
explosive gas is always obtained. For then the flame
can always go through the whole mass and burn it at
once, whereas otherwise the burning can only take place
where the air ** reaches."
P. Yes, you made that clear before.
M. I made something else clear to you too. How
can the hydrogen flame be made still hotter than it is at
148 CONyERSATlONS ON CHEMISTRY.
present? Do you remember what I told you about
burning in air and in pure oxygen?
P. Yes, I know: if you were to burn hydrogen with
pure oxygen, the nitrogen in the air wouldn't need to
be heated with it and the flame would be hotter.
M. Right. How would you do that?
P. I would let the hydrogen flame burn in a flask
which contained oxygen.
M. Quite right, but not convenient. A very high
temperature is obtained if oxygen is blown into the hydro-
gen flame.
P. But how can that be done?
ikf. Well, we could take an empty india-rubber balloon
and fill it with oxygen and then press it ; then the oxygen
would stream out of the opening. But I will show you
how a proper gasometer is made. I have here two
very large flasks which are provided with corks, in each
of which are two holes. Through one hole a siphon
of glass goes to the bottom, through the other, a short
bent tube (Fig. 28). Both siphons are connected by a
piece of rubber tubing, and one flask is filled with water.
P. I can't quite see what use all this is going to be.
M. Just notice: I now connect an oxygen apparatus
(page 84) to the bent tube of the flask filled with water,
and place the other flask at a lower level. If I make
oxygen by heating, it goes into the upper flask and the
water runs through the india-rubber tube into the lower
one.
P, That is pretty.
M. So now the higher flask is full of oxygen. I take
the oxygen apparatus off, and close the rubber tube with
a clip.
P. What is a clip?
OXYGEN AND HYDROGEN.
149
M. It is a wire spring which squeezes the rubber
so as to close it. Such a chp is very easy to put on, and
often closes a tube better than a stop-cock, so that it
is very often used in chemistry.
P. I like that, it is so simple and useful.
M. Now we can let our oxygen stream out whenever
we wish. I only require to raise the flask containing the
Fig. 28.
water, and the oxygen, under the pressure of the water
from such a height, streams out if I open the cHp. If
I close the cHp, the stream stops again. If I don 't want
oxygen for some time I put the higher flask low again
and there is no more pressure.
P. I Hke that.
M. Now I fix my glass tube with the platinum-point
on the gasometer and fasten it so that the point pro-
jects into the flame of the spirit-lamp. I let the oxygen
ISO CO^yERSATIONS ON CHEMISTRY.
pour out, so that the flame will be blown to the side; at
the same time it will be small and pointed and very hot.
P. It only looks a little brighter.
M. I hold a thin piece of platinum wire in it; you
see that it not only glows white-hot, but soon melts.
Now a pretty round ball is formed at the end of the
wire, and if I heat it longer it will fall off.
P. It is so bright that one can hardly see. But you
were going to show me the temperature of the hydrogen
flame.
M. In this flame it is the hydrogen of which the spirit
mainly consists which is burnt. But, to produce a good
hydrogen flame, we must make our apparatus somewhat
larger and more powerful. . As it is at present, it gives
out a lot of gas if fresh acid has been put in, but soon less,
and a regular flame cannot be obtained. We will make
an apparatus that will give us just as much or as Httle
gas as we need.
P. I am curious to see how you will make that appa-
ratus.
M. I take two flasks with the right corks and tubes,
exactly the same as the oxygen-gasometer, only that I
take rather smaller flasks. One of them is filled with
zinc and in the other is dilute hydrochloric acid; the
latter is raised higher than the former. When I open
the cHp which is on the zinc flask, the hydrochloric acid
comes through to the zinc, and hydrogen is evolved.
P. But nothing comes.
M. The siphon is not filled yet, and so cannot work.
But I only need to blow down the tube of the hydrochloric-
acid flask. Now it comes.
P. Yes, now the acid is bubbling. But why did you
first put a layer of pebbles in the zinc flask?
OXYGEN AND HYDROGEN. l^^
M. That you will soon see. I close the clip which
lets the hydrogen out. What do you see?
P. The acid goes back again through the siphon into
the upper flask. Ah, now I understand. The hydrogen
which can't come out any more presses the acii out of
the lower flask into the upper one.
M. Quite right. However, as not all the acid can
have gone back, because the bottom is uneven, some
must have stayed behind, which would work further on
the zinc. But now this residue merely remains with the
pebbles.
P. That is pretty: a regular automatic machine.
M. I first test, my hydrogen to see if it is pure, and
then light it. I open the clip so as to give a fairly large
flame. For this purpose the cHp is provided with a
screw (Fig. 29). Now I bring up the platinum tip with
Fig, 29.
the oxygen, and you see how small and pointed the
flame becomes. A piece of platinum wire melts far more
easily than before. A steel watch-spring that is heated
at the end, first glows white-hot, and then burns with
lovely streaming sparks, as in oxygen. A piece of chalk
that I have pointed begins to glov/, and gives such a
brisfht white light that it looks like sunshine.
p. That is a pretty firework!
M. It shows you that the flame of pure hydrogen and
oxygen, or, to put it shortly, the oxyhydrogen blowpipe,
is really uncommonly hot.
152 CONVERSATIONS ON CHEMISTRY.
P. That must be about the highest temperature that
can be reached?
M. No; the flame is only 2000° C, while between the
charcoal- points of an electric-arc lamp over 3000° C. is
reached. But still it is a very high temperature, which
our furnaces never nearly attain.
P. What a lot I have seen and learned to day!
19. WATER.
M. To-day we shall study water itself, after having
learnt about its constituents and formation. You know
that water occupies the greater part of the earth's surface.
P, Yes, about five sevenths.
M. Now, the water which forms the oceans, lakes,
and rivers is not by any means pure water, but contains
many other dissolved substances.
P. I know that sea-water contains salt; but I don't
know anything about other water, or that other sub-
stances should be in it.
M. How do you know of the presence of salt in sea-
water?
P. By its salt taste.
M. Quite right. Then do all other waters taste the
same, rain-water and spring- water, for example?
P. No. I once tasted rain-water; it had a horrid taste.
M. Well, you must conclude from the difference of taste
of these other waters that they contain different substances.
Here you have a specimen of pure water; just taste it.
P. It tastes just as nasty as rain-water. How is pure
water ma.de?
M. By distillation. That is to say, it is first changed
HEATER.
153
into steam, and this steam is cooled again till it changes
into liquid water.
P. But how is the water purer for that?
M. The impurities which are contained in ordinary-
water do not change into steam, as they are not volatile.
I take some ordinary drinking water and add ink to it, so
that you can see the impurity quite distinctly; if I distil
this black liquid, a pure and clear water comes over.
P. I should like t3 see that. How is it done?
M. In different ways. We will do it in the simplest way
first. I put a cork with a hole in it into this thin-walled
flask, and by this means attach to the flask a tube which
is rather sharply bent over. I pour my black water into
the flask, and heat it till it boils (Fig. 30).
Fig. 30.
P. Now there is some steam in the tube, and now a
drop of water is running down ; it is really quite clear.
M. We will put another flask over the lower end of
the tube, to collect our distilled water.
P. Now this flask is being covered up inside with mist
and now the steam is coming out, and isn't condensing.
M. What is the reason of that?
P. The flask has become too hot, and can't cool the
steam any more.
154 CONyERSATIONS ON CHEMISTRY.
M. Quite right. To distil properly, we must provide
a cooler. I can do that simply: I place a dish of cold
water so that the flask stands in it; that will keep it
cool.
P. But if the water gets warm ?
M. Then we must stop. You have here an important
fact, which is a great question in chemistry: all work
must be so arranged that it can be carried on continu-
ously. In order to do this, what is required must be
dehvered regularly, and what is superfluous must be
regularly got rid of. What is being used up here ?
P. The water, which changes into steam.
M. Right. Besides that, the heat, which is necessary
to make steam. And what is superfluous?
P. The warm water in the dish. That could be changed
by letting it out through a siphon and filling it from
above.
M. Good; and the water which has distilled over could
be replaced in the flask by means of a funnel.
P. But the steam would escape.
M. The tube need only be dipped under the water,
and then it is closed. But our cooling could be improved,
because if our receiver is only half in water, the upper side
remains uncooled, and the steam won't be completely
condensed.
P. It must always be turned round so that the cool
side is at the top.
M. Then a man or an apparatus is needed to turn it.
We must have a cooler that does all that is necessary itself.
P. Then water can be allowed to run over the top
side, as well as run out at the bottom.
M. That is better. But there is still a difficulty: the
running cold water will mix with the warm water in
IV/ITER. 155
the dish, and a great deal of cold water is needed. Can't
that be bettered?
P. You are asking a great deal !
M. If a technical or scientific exercise has to be done,
you must never be contented with what you have reached,
but must always ask: Can it not be made better? And
if you find a fault or incompleteness, you must always
ask: How can I improve it?
P. I can't do it.
M. It is possible with this condenser (Fig. 31). It is
made with an inner tube for steam and an outer jacket
Fig. 31.
for cold water, which can be made of tin. The jacket
is provided with doubly bored corks at both ends; the
steam-tube passes through one opening, and short tubes
are stuck through the others, of which the lower one
is for letting in and the upper one for letting out the
cold water. A screw clip is used to regulate the amount
of water let in; the hot water is let out at the top.
P. Why must the cold water come in from under-
neath? I thought it would cool better if you let the cold
water cool the steam at the upper end.
M. It would be just the opposite, because it would
be wasteful, since warm water is lighter; it would always
rise upwards and mix with the cold water. But when
15<5 CONyERSATIONS ON CHEMISTRY.
the cold water comes from underneath it serves to con-
dense the remainder of the steam. It gets hotter towards
the top, and gives up its heat as thoroughly as possible
for coohng purposes, for the steam which enters above
is condensed by the nearly boiling water, and in this
way the coohng water is most thoroughly used, because
1 useless mixing of the cold water and warm water is
ctvoided.
P. I begin to see how many things you have to think
of in setting up a small apparatus.
M. This is the first instance you have had of the
principle of opposing currents. While the vapour streams
from above downwards, and loses its heat more and
more, the cold water streams from below upwards, and
absorbs that heat regularly. You will later on get to
know a great number of other cases where the same
principb of opposing currents is employed. Its use is
accompanied with the greatest possible economy.
P. I can't quite understand that, but I'll try to remem-
ber, so that I may look out for other instances of the
same kind.
M. Now we have collected some distilled water. You
can convince yourself that it has exactly the same taste
as what I gave you before, and has not the least taste
of ink.
P. Why does it taste so bad? Well-water has no
particular taste, yet it is pleasant to drink.
M. Because we have always been used from our child-
hood to drink well-water, in which certain foreign sub-
stances are contained, and have grown accustomed to
it. Pure water makes a different impression upon our
nerves of taste from well-water and we call it unpleasant.
Now we will make a wash -bottle.
IVATER.
157
Fig. 32.
P. What is a wash-bottle and what is it used for?
M. We must use pure water for our chemical experi-
ments in order not to mix other substances with our
solutions. We keep this water
in a vessel in order that we
may conveniently use it. First
I cut off a piece of glass tub-
ing half as long again as the
height of this flask ;^ and then
a short piece. I hold the long
bit in the flame and turn it
round till its edge softens; it
contracts, and when the open-
ing is reduced to half a milli-
metre I let it cool. Then I
bend the short tube to an
obtuse angle, and the long one, after the end has cooled,
to an acute angle; and lastly I round all the ends. And
now I bore two holes, in a cork that fits the flask, stick
the tubes through the holes, and my wash- bottle is ready
(Fig. 32). Now we will fill it with distilled water, after
having washed it out several times.
P. What is the use of all that?
M. When I blow into the short tube, water issues out
of the longer one in a thin stream which I can direct
where I like. And if I need more water, I turn the
flask upside down and a pretty large stream pours out
of the short tube.
P. It looks to me as if you had taken a great deal of
trouble for very little purpose.
M. Not at all; for, by the use of the wash-bottle, my
daily work is made so much more easy and certain that
my trouble is soon rewarded. Every mechanic takes
^58 CONVERSATIONS ON CHEMISTRY.
care to provide himself with the best possible tools, even
though they are dear; he is repaid with ample interest
because he is able to produce more and better work in
the same time. For the chemist a wash-bottle is a suit-
able tool.
P. But my father has told me that Benjamin Franklin
once said that we ought to be able to bore with a hammer
and saw with a gimlet.
M. That is not bad advice; it means that one should
be able to adapt oneself to anything. But there is a
great difference between getting over a difficulty once,
and regular work. For example, I njight write with this
match dipped in ink, if I had no pen, but as I can write
better and quicker with a pen, I prefer it. But we have
forgotten our water all this time. What is the colour
of water?
P. I don't think it has any. It is colorless.
M. Yes, in thin layers it appears colorless, but in thick
layers pure water is blue.
P. What is the reason for the difference ?
M. Water is so faintly colored that in thin layers the
color is not recognizable. But you learned long ago that
the color is more distinct the thicker the layer. Pure
water in a white bath shows the blue color distinctly.
P. The next time I take a bath I will look out for
that. But the water in the river is not blue, but brown.
M. The reason for that is that the water in the river
contains foreign subtsances, the color of which is brown.
Sea-water is generally pure, and has a blue color; but
if it is mixed with brown substances the mixture looks
green.
P. But sea- water is not in the least pure, for it cc n-
tains salt.
IVATER.
159
M. Quite right; but salt is colorless, and so it doesn't
alter the color of the water. What is the density of
water ?
P. I remember that. Its density is i, for it serves as
the standard of density.
M. Good; that is, its density at 4°C.: at all other
temperatures it is less. While all other substances
expand by heating, water contracts between 0° and + 4° C.
And above that temperature it expands.
P. I should hke to see that. '
M. There are several ways of showing it. Take a
wooden bucket, bore a hole in the side near the bottom,
and cork a thermometer
into the hole. Then fill
the bucket with ice-water
in which pieces of ice are
floating, and let it stand
(Fig. 33). After some time
the thermometer below will
show the temperature + 4°
C, while another thermom-
eter dipped in the water at the top will stand at 0°.
Explain that to me.
P. Because water at 4° C. is heavier, and must collect
at the bottom.
M. Something more might be said about that, but it
is right in the main.
P. How would this do; would this not be simpler?
If water were enclosed in a thermometer-tube, it would
contract between 0° and 4°, and then rise again. Could
not a water-thermometer hke that be made ?
M. Yes, of course. Here I have a glass tube of pretty
Fig. 2>Z'
i6o
CONVERSATIONS ON CHEMISTRY.
F&
narrow bore, about half a millimetre. I heat the
end till it meks, and blow in; I make a bulb
just like a soap-bubble. I fasten on the
upper end a cork with a wider piece of tub-
ing, which I fill with water (Fig. 34). I first
warm the bulb slightly, and air-bubbles escape
through the water above. Then I cool it
down again, and some water is sucked into
the bulb. I boil this water, and when I take
away the flame the water rushes into the
bulb and fills it. Generally a small air-
bubble remains, but that is easily removed
by heating the water, and then coohng it;
the bubble is pushed out, and on cooHng the
water fills the tube.
P. But how can a scale be fa^ened on ?
M. I take a piece of an old millimetre scale
or some divided paper, or something of that
sort, and stick it on the tube with sealing-wax. After
my water-thermometer has taken the temperature of
the room, I remove the upper tube. Now I will trust
the apparatus in your hands, and also a thermometer.
Tie them both together so that you can read both scales
easily, and place them in a large dish of water. Now
notice where the mercury in the thermometer stands and
where the water stands. Now put some ice in the water
so that the temperature sinks about a couple of degrees,
and stir it for at least five minutes till the water-ther-
mometer has got steady, and write down the readings
you find. Go on till the temperature is nearly 0°. Tell
me to-morrow what you have found.
6
Fig. 34.
P. I'm afraid that what I have done is not worth any-
fVATER. i6i
thing. I played the whole afternoon with the thermom-
eter, but I couldn't find out that the volume of water was
smallest at 4°.
M. What did you find?
P. That the water sinks, to begin with, as the ther-
mometer falls, but at about 8° it stops, and if I cool it, it
rises again. I always get 8° as the temperature of the
smallest volume.
M. What do you think is the reason for that ?
P. I didn't think of looking for a reason. I only
thought that I had read it wrongly, but I always got the
same.
M. Then your readings were right. What quantity
were you measuring?
P. The volume of the water.
M. No, you were only measuring the position of the
water, and drawing a conclusion from that as to its
volume. Before you can draw a conclusion from the
position of the water as regards its volume, you must
make sure that the capacity of the thermometer-bulb
always remains the same. Are you quite sure of that?
P. Let me see. Yes, I always found the same posi-
tion at the same temperature.
M. Very good. But from that you can only conclude
that at the same temperature the capacity was the same.
Do you see now?
P. You mean that the glass of the bulb had expanded
by heat? That couldn't make any difference, because
the glass is so thin that it makes only a very small
fraction of the volume of the water. And the small
expansion of this small volume couldn't make a great
difference.
M. You have made a mistake in reasoning. You
1 62 CONyERSATlOm ON CHEMISTRY.
have supposed that the alteration of the volume of the
glass had to be considered? That is not true. You
should have considered the increasing volume of the
glass bulb, which is the same as that of a solid ball of
glass of the same size as our thermometer-bulb, and is
nearly as great as the expansion of water.
P. But the ball is not solid.
M. Think of a solid ball heated uniformly to any
high temperature; will the interior be in a state of strain
or in equilibrium?
P. I think it will be in equihbrium, for it expands
uniformly.
M. Right. Now think of this ball as consisting of
a number of hollow balls fitting each other accurately,
like the layers of an onion ; would there be any difference
if such a ball were heated?
P. I see no reason. Ah, now I understand: the out-
side layer would expand exactly as if the inner layers
were not there, just as if it were a sohd ball. That's
very ingenious.
M. Now you see the reason why you found the point
of the smallest volume of the water too high. If the
water hadn't expanded at all it would have sunk in the
stem, because the volume of the bulb would have increased.
It is only when the expansion of the water is exactly
even with that of the glass that it remains stationary
in the stem; and that is at 8°. As you see, you have
been examining the difference between the expansions
of water and of glass, and in order to find out the former
you must know the latter, but that is not easy to find out.
P. Oh, bother! I thought I was doing the thing well
and I have been wasting my time.
M. Not wasting, because you have learnt how much
ICE, 163
there is to think about for every experiment before you
know how to interpret it.
20. ICE.
M. Yesterday you learned some of the properties of
water; which do you remember best?
P. The greatest density of water and the experiment
about that. I tried it with a pail and it came out all
right.
M. Good. It is important in nature that the density
of water has its greatest value at 4°; that is called the
temperature of maximum density.
P. Why should such a small difference be so important ?
M. When still water, for instance a lake, is cooled on
the surface, the colder water sinks till the whole of the
water has reached the temperature of 4°. Then the
cold water stays above till it freezes, while below the
temperature 4° persists, just as in your experiment with
the pail (page 159).
P. Then fish aren't so cold, after all?
M. That is of no great importance; but if it were not
the case, ice would be deposited at the bottom of the lake
and it would freeze through and through, instead of
being coated with ice only on the surface. The fish would,
of course, die, and in spring it would be much longer
before all the ice melted. In quickly running rivers where
the water is thoroughly mixed it sometimes happens in
a hard winter that all the water is cooled below zero, and
then ground-ice is formed, which floats to the top when
its mass has sufficiently increased. /
164 CONyERSATIONS ON CHEMISTRY.
P. I should have thought that the lake would become
covered with ice, for ice floats upon water.
M. There is another condition which protects lakes from
freezing. This brings us to the properties of ice. You
know that water changes to ice at 0°. But now I will
show you that that isn't always the case. I mix some
pounded ice with a little salt, and the temperature falls
below 0°, and is lower the more salt I mix with the ice.
Now give me your water-thermometer and the mercury-
thermometer. The temperature of my cold mixture
is — 5°. I'll put the bulb in and let the water cool itself
down.
P. It will freeze and the bulb will be sure to burst.
M. Then you can blow a new one. But you needn't
be afraid; it won't freeze.
P. Why is that?
M. As long as there is no ice present water can be
cooled considerably below 0° without freezing. Only,
when you bring it in contact with some ice, the water
becomes solid.
P. Why .is that? — I beg your pardon, I must ask
the question differently : What else is it connected with ?
M. That is a difficult question to answer. Now
remember that the temperature 0° always persisted when
water and ice were simultaneously present. If you
cool water alone below 0° ice may be formed, but it is
not necessary that it should be. That is a general state-
ment; even though the conditions are present for the
formation of new substances or forms, these do not gen-
erally appear of their own accord, but the point of change
may be more or less exceeded. Only, this becomes im-
possible when these new substances are present, for they,
being already in existence, increase.
ICE. 165
P. That is no explanation; it is only a description.
M. Quite right. You know now under what cir-
cumstances such phenomena become manifest, and
what their relations are. What more do you wish?
When you have learnt more about chemistry you will
get to know of other relations, and be able to look at
these phenomena from many points of view. That is
all that we can hope to learn from science, and it is
surely enough. In order that we may be able to talk
of such things in future, I may tell you that what you
have seen with water is supercooling; a more general
word is supersaturation.
P. I see I have a great deal to learn still.
M. So have we all. Then ice floats upon water; what
conclusion can you draw from that?
P. That ice is lighter than water.
M. Do you mean that water loses weight when it freezes ?
P. No. Water which is displaced by ice weighs more
than the ice.
M. When the ice is completely immersed in the water.
Or in other words, when water freezes the resulting
ice occupies a greater volume than that previously occu-
pied by the water. There is a good deal of difference:
ten volumes of water give more than eleven volumes of
ice. That is a pecuHarity of water. Most other sub-
stances contract on freezing, so that the solid sinks in the
liquid.
P. Has that any connection with the expansion of
water below 4°?
M. That is a question which has given rise to much
speculation. But no satisfactory explanation has yet
been found. Have you ever seen water beginning to
freeze ?
i66
CONyERSATlONS ON CHEMISTRY,
P. Oh, you mean when only a httle of it is frozen?
Yes, long needles appear on the surface. I have often
seen it on the puddles.
M. These are crystals, for ice is a crystalline body.
P. I know, I have often seen large snow- crystals.
They look like stars with six rays, or six-sided plates.
M. Quite right. Here are some photographs of snow-
crystals (Fig. 35). The frost on the window-pane con-
sists of ice- crystals.
Fig. 35.
P. But their shape isn't regular.
M. Because the water freezes too quickly on the pane
for the crystals to have time to form. But sometimes
where the pane is almost quite clear you will see pretty
regular crystals which have deposited slowly from the
water in the air.
P. Then hoarfrost also consists of crystals?
M. Yes. You have seen them glistening in the sun,
which is reflected on their surfaces. A sheet of ice on a
frozen pond can also be shown to be crystalHne. Ice is
as blue as liquid water.
P. But snow is quite white! Stop, I know why;
because it is so finely divided (page 9). And I remem-
ICE. 167
ber that large blocks of ice that you see on the street
look quite blue.
M. Large masses of ice called glaciers slip down from
high hills which are covered with everlasting snow;
when they move they split, and in the cracks, or crev-
asses as they are called, they look beautifully blue.
P. I suppose because the light has to penetrate through
thick layers of ice.
M. Quite right. Now we will look at some ice melt-
ing. I take a thick iron plate and lay it upon a tripod
above the spirit-lamp. Now I take two similar beakers
or flasks and put some ice into the one, and into the
other an equal weight of water at 0°. I place these
beakers symmetrically on the plate, so that each gets
the same amount of heat from below; and in each I
place a thermometer. Now we can proceed with the
experiment.
Fig. 36.
P. What is to be learnt from that?
M. That ice can absorb a lot of heat without becom-
ing warmer.
1 68 CONVERSATIONS ON CHEMISTRY,
P. How can that be?
M. Look here: the thermometer in the water has
risen from o° to 20°. The one in the ice is still at 0°.
P. The reason for that must be that ice^ is present with
the water, and therefore the temperature must stay
at 0°.
M. Quite right; as much heat must have entered the
ice as was necessary to raise the water from 0° to 20°,
and yet the ice is no warmer. What has happened to
the ice ?
P. Some of it has melted. So heat is really used up
by the melting of ice?
M. Exactly. What is heat?
P. One kind of energy or work. So it requires work
to change ice into water.
M. Quite right. Before people had acquired a con-
ception of energy, they were very much surprised at this,
and said that although heat was not perceptible to the
thermometer it must nevertheless be present, and only
lay concealed; they, therefore, called this heat latent
heat, from lateo, I lie hid. Even now this name is
used, although the former false conceptions have been
replaced by correct ones.
P. I don't quite understand that.
M. You know that in general, work or energy is used
up in producing a change of state; so it is here. For
example, when you grind a piece of sugar to powder you
can't do that without work, just as when you break
a rod or bend a wire. In the same way melting requires
work, and this work is derived from simple addition
of heat.
P. Can the work be done in any other way?
M. Certainly. If two pieces of ice at c° are rubbed
ICE. 169
together they melt. Now the ice has melted and the
thermometer has risen to a little above 0°. The other
thermometer shows nearly 80°. Now notice. The
amount of heat required to raise i gram of water through
1° is called a calory, abbreviated cal. To raise i gram
of water from 0° to 80°, 80 cals. are required; to raise
200 grams of water to 30°, 200X30 = 6000 cals. are re-
quired. The amount of heat is measured by multiply-
ing the rise of temperature by the weight of the water.
P. I understand that. But if the water grows colder?
M. Then heat, equal to the product of the lowering of
temperature multiplied by the weight of the water, has es-
caped. Now the water has become 80° warmer by absorb-
ing the heat required to melt an equal weight of ice; so
that each gram of water has absorbed 80 cals. and each
gram of ice exactly the same quantity. And it follows
that each gram of ice requires 80 cals. in order to melt
to water at 0°. In other words, 80 cals. are the work
of melting, or the heat oj jusion of ice. The old name
which I explained to you before is the latent heat of water.
P. But this number refers to i gram of ice.
M. Quite right. Such numbers are generally referred
to unit of weight, because then it is only necessary to
multiply by the weight in order to find the value for a
given quantity. Let us make an application of this.
We will weigh 500 grams of water into a beaker, and
after measuring the temperature with a fine thermom-
eter we will drop in a piece of ice. The temperature
is 18.7° and the ice weighs 34 grams. Now I put the
ice into the water, and stir it carefully with the ther-
mometer until all the ice has melted. The thermometer
has fallen to 12.4°. You can calculate the latent heat
of water from this.
17° CONVERSATIONS IN CHEMISTRY,
P. I'll try. 500 grams of water have lost 18.7 — 12.4=
6.3O so that 500X6.3 = 3150 cals. have been used. That
heat melted 34 grams of ice, so that each gram took
93 cals. Is that right?
M. Pretty nearly, but not quite. By heat of fusion is
meant the heat required to change i gram of ice at 0°
into vi^ater at 0°. The ice-water, however, gets heated
up with the rest of the water to 12.4°, so that you have
calculated the heat of fusion higher than it should be.
P. Yes, I see that, but how can it be corrected?
M. By bringing everything that happens into the
calculation. You were right in supposing that 500 grams
of water had lost 500X6.3 = 3150 cals. But of this
34X12.4 = 422 cals. have been used in warming the
melted ice, and only the difference, 3150—422 = 2728,
has been used for melting the ice. This difference
divided by 34 gives 80 cals. as the heat of fusion of ice.
P. I see again that making experiments is much
easier than drawing the right conclusions from them.
M. But even here we haven't taken everything into
consideration. We have paid no attention to the fact
that not only the 500 grams of water were cooled down,
but also the thermometer in the beaker. Then we
just noticed that the beaker with the cold water is grad-
ually warming itself in this room, so that while the ice
was being melted, heat was entering from outside, and
the lowering of temperature was therefore too small.
Even that is not all that we ought to have considered,
but I will refrain from adding more so as not to confuse
you.
P. I'm rather muddled as it is, and can't understand
how there are people who know all these things and can
do them correctly.
STEAM. 171
M. You can neither use a lathe nor paint, and while
you were learning to cycle you found that very difficult.
To make correct measurements is an art in itself which
has to be learnt, and no one ever learns it thoroughly.
Exact measurements prove the heat of fusion of ice to
be 81 cals.
21. STEAM.
M. To-day we shall speak about steam.
P. Water again! If we are going to spend as much
time over other substances, I shall never have done
with chemistry.
M. Water is only an example by means of which
we learn the behaviour of substances under different
circumstances. All the so-called laws, for example those
relating to melting and solidifying, are the same with
other substances, so that you don't need to learn them
again. -=,_
P. But why did we choose water as an example?
M. Because water has been more studied than any
other substance and is therefore best known.
P. But why was water so carefully studied?
M. Because it occurs in such large quantity on the
earth. Just think how differently the surface of the
earth looks when the temperature is below 0°. Of
course the reason is that water freezes at 0°. The dif-
ference isn't merely the appearance of ice and snow, but
also the temporary cessation of life in plants, caused by
the fact that the sap can no longer flow.
P. Yes, I see that w^ater affects almost everything.
M. Besides, since water is so abundant, it is easier to
obtain purer than other substances, and so it is especially
172 CONyERSATIONS ON CHEMISTRY.
adapted as a standard with which to compare proper-
ties. You have learned this use of water already for
the thermometer and for density; and for many other
properties water serves as a standard. .And that of
itself is a reason for getting to know the properties of
water more thoroughly than that of other substances.
We shall therefore take up, to begin with, the boiling of
water.
P. Is there anything special about that ? I have been
taught that water boils at ioo° C. whether a large or a
small flame is below it.
M. Let us see. I boil some water in a flask and close
the mouth with a cork while it is boiling; what will
happen ?
P. The pressure of the vapour will rise and the flask
will burst.
M. Quite right, so I take the flame away and let it
cool down. But as it cools too slowly, I will pour some
water on the flask. What do you see ?
P. How funny! The water is beginning to boil again.
M. I pour more water on it and the boihng begins
again. Now it is so cold that I can hold it in my hand
without burning it; the water can't be hotter than 50°
and still it boils whenever I pour cold water on the top
of the flask.
P. I can't believe that in the least.
M. Why not? You must beheve what you see.
P. But I was taught that water boiled at 100°, and
now it's boihng at a much lower temperature.
M. Well, what conclusion do you draw?
P. That water can boil at all possible temperatures.
But that is nonsense.
M. Why?
STEAM. 173
P. Because the water always showed the tempera-
ture 100° whether the flame was big or small.
M. Quite right. But when you see that a phenom-
enon changes you must conclude that some condition,
closely connected with the phenomenon, has changed.
Now think what is the difference between the boiHng,
now and before?
P. Heating made the water boil, but now it boils
when it is cooled.
M. It can't be only cooling, because then water would
always keep on boiling when you took away the flame.
Is there nothing else you can think of?
P. Yes, you corked the flask. But how can a cork
make the water boil?
M. Take the cork out.
P. That's not easy. And it hisses as if air were being
sucked in.
M. So there is a vacuum in the flask. Now think
why.
P. I begin to see. The steam blew out the air, and
then you corked the flask so that no more air could
enter.
M. Quite right. The flask contained only water and
steam, and when I poured cold water on it, the steam
was condensed, the pressure was lowered, and the water
was obliged to boil.
P. But does water really form steam at every tempera-
ture if the pressure is lowered?
M. Water boils at every pressure, and a definite
temperature corresponds to each pressure. It boils at
100° only when the pressure is exactly that of the at-
mosphere. On high mountains where the pressure' is
much lower boih'ng water is not hot enough to cook meat
174
CONVERSATIONS ON CHEMISTRY
P. I should like to see that.
M. I can show you something like it. I close the
flask with a perforated cork through which passes
a doubly-bent glass tube,
one of the limbs of which
is 80 cm. long (Fig. 37).
Its end dips in a basin
of mercury. Now I heat
the flask again; and you
can hear the air bub-
bling through the mer-
cury. Now the noise
changes; it rings almost
like a metal.
P. What is the reason of
that?
M. The steam is now
almost quite free from air,
and when it passes into the
cold mercury it suddenly
changes to liquid water,
and the sides of the bubble
hit each other. As long as
air was present the sides
couldn't come together, but
now it is metal hitting
Fig. 37. metal. Now I take the
flame away and you see that when I pour on cold water,
the boihng begins again.
P. What is the use of the tube that dips in the mercury ?
M. Notice what happens when I pour on cold water,
the flask.
P. At first the mercury goes up quickly, and then it
^
J
STEAM. 175
falls when the boiling begins; but I see it stands higher
than at the beginning.
M. Now you see everything happens as I told you it
would. The higher the mercury is sucked, the smaller
the pressure on the inside of the flask. It was highest
immediately after I had poured water on it; but when
the water began to boil steam was formed, which again
filled the space and increased the pressure, and the mer-
cury fell.
P. But why did the level of the mercury always become
higher each time?
M. Because the water in the flask became colder
each time I poured water over it, and its vapour pressure
decreased. It boiled only when the pressure was made
still smaller.
P. So that boiling takes place when the pressure on
the water is less than the vapour pressure. I see you
nodding, so that's rignt. But what is vapour pressure?
There is never anything but vapour in the flask.
M. Imagine an empty space; of course there is no
pressure in it. Now introduce some water; part of the
water changes to steam. This goes on till the space is
filled with steam to a certain extent, and then evaporation
stops. And vapour is formed till it has a certain definite
density in the space, and exerts a definite pressure.
The density and the pressure depend only on the
temperature. At 0° the pressure is very small and could
only raise the mercury 4 mm. high; but at 100° it is
so great that it can overcome the whole pressure of the
atmosphere.
P. And above 100° ? Can water be made hotter?
M. Certainly; only then the pressure must be increased
by confining the vapour. As you know, this occurs in
1 7 6 CON VERS A TIONS ON CHE MIS TR Y.
a boiler. If the pressure is twice as high as the atmos-
pheric pressure, the temperature of the water is 121°.
And if its temperature is 180° the pressure is ten times
as great. The steam-engine is contrived to utihze this
pressure. You can see on every steam-boiler an ap-
paratus which looks something like a clock-face and is
called a gauge, or a manometer; it measures the pres-
sure.
P. I have often seen it. Why is the pressure meas-
ured in pounds?
M. It means pounds per square inch. The pressure
of the atmosphere, that is, the pressure which the air
exerts upon the surface of the earth, is 16 lbs. per square
inch. But steam is used for heating, as well as for driv-
ing, steam-engines. Do you know why?
F. Because its temperature is 100°.
M. That is not all ; it can give up far more heat than
water at 100°.
P. That is the same as with water and ice.
M. Quite right. To change water at 100° into steam
of the same temperature requires a great deal of work
which can be added in the form of heat. We can make
an approximate measurement. We will first heat a
known weight of water over the lamp, and by measuring
the time we will calculate from its amount and from the
rise of temperature how much heat the lamp gives it
per minute. Then we will boil the water over the same
lamp and measure the time that it boils; then we will
weigh it again, and from its loss we will find how much
vapour has been formed; then we can calculate * how
many calories are required to evaporate i gram.
P. I should like to do that. What sort of dish shall I
take?
STEAM, 177
M. Take a flask; we shall weigh out 200 grams of
water. Now we will put a thermometer into it; its
temperature is i3°. The lamp has been burning for
some time and the flame has become regular; I place it
below the flask, and let it burn for fifteen minutes. Now
what is the temperature? Remember to stir before you
read.
P. 78°. That is 60° in fifteen minutes or 4° a minute.
As there were 200 grams of water, the lamp gives 800 cals.
a minute.
M. Quite right. Now the water begins to boil and
I look at my watch again. In ten minutes I take the
lamp away and let the flask cool. On weighing it, it
has lost 14 grams. How many calories does that require
to produce i gram of steam?
P. Ten minutes for 800 cals. makes 8000 cals., and
dividing by 14 gives 571 and a fraction.
M. Fairly good. The right number is 537 cals.
The reason why we found it too high is that the flask
while it was at 100° has been losing more heat than
while it was being heated up from 18° to 68°o
P. Yes, I can quite imagine that all kinds of things
must be thought of in order to get correct numbers.
M. That is true; the measurements are much more
difficult than with ice. But we won't trouble about that
at present. As you see, the heat of evaporation of water is
nearly seven times as great as the heat of fusion of ice.
P. Yes, the heat of fusion was 81 cals.
M. For this reason steam can be used to convey heat
from one place to another without the necessity of
carrying much weight. The vapour is produced in a
boiler, and led through pipes to where the heat is wanted.
In schools and public buildings, heating with steam is
1 7^ CON VERSA TIONS ON CHE MIS TR Y.
often adopted, and on turning a stop-cock, you can
make it either cold or hot.
P. But when the steam has given up its heat, it goes
back to Hquid water. What becomes of the water?
M. It is led back to the boiler. The water makes
a circular tour through other pipes; but the heat
goes from the boiler to those places where it is wanted
and stays there. It is much the same as when the
piston of a locomotive conveys motion from the engine
to the wheel, and again comes back; but the work stays
there.
P. Is there steam-heating in railway carriages ? One
often sees steam escape between the carriages in winter.
M. Yes, the steam which has escaped from the cylinder
of the locomotive is used for that purpose, after it has
done its work. — So now we know water in all its three
forms, but we have not nearly done with it. Of its
other properties, perhaps the most important for us is its
power of dissolving substances. Do you remember what
you learned about that?
P. That water becomes saturated when it dissolves
anything.
M. Be more exact.
P. When you put into water anything it can dissolve,
only a definite quantity goes into solution. And when
the water can dissolve no more, it is said to be saturated.
M. But suppose you were to take three times as much
water?
P. Then three times as much substance would dissolve.
M. Quite right, but that is only at some definite tem-
perature. If you were to heat the solution —
P. Then it would dissolve more.
M. That is not always right. It is true that most
STEAM. 179
substances behave in that way, but there are some that
dissolve equally well at different temperatures. Common
salt is such a substance; it is nearly equally soluble in
hot and cold water.
P. Are there any substances which dissolve better in
cold than in hot water?
M. There are, but they are rare.
P. Which substances dissolve in water, and which don't ?
M. Strictly speaking all substances dissolve in water;
but there are many which dissolve so slightly that very
accurate tests are necessary to find out that they do.
P. Surely glass is not soluble in water?
M. Yes, indeed; although it is only very sparingly
soluble.
P. How can I see that?
M. Take some beet-root juice and put it on a piece
of glass. It stays red. But if you powder the glass in a
mortar along with beet-root juice, the juice turns blue
and green. The reason is that the glass dissolves and
acts upon the juice so as to turn it green.
P. Why must the glass be ground up in a mortar?
M, The solution takes place faster when the surface
is increased.
P. I hadn't thought of that. But stones don't dissolve
in water.
M. All river- and well-waters contain dissolved sub-
stances. You can see that from the deposit in the tea-
kettle where the dissolved substances settle out as a
grey crust which is called fur.
P. Yes, I have just seen them removing that fur. It
stuck very tight.
M. Well, these dissolved substances came from the
rocks through which the water flowed before it reached
l8o CONyERSATIOm ON CHEMISTRY.
the surface. For, lo begin with, the water was pure
distilled water.
P. How could that be? Who distilled it?
M, Well-water comes from rain which falls on the
surface of the earth, sinks in, and collects in deeper
places. Where does rain come from ?
P. From the clouds.
M. Yes, and clouds are formed by the condensation of
water- vapour out of the air. So that rain-water is really
distilled water, indeed quite freshly distilled. When you
see it it has generally run over a roof, and carries all
the dirt with it that has collected on that roof since the
last shower. How does water come into the clouds?
P. It evaporates from the surface of the earth, and
is driven about by the wind.
M. That is right so far, but to evaporate it recjuires
heat, and you have just measured how much. Where
does it get the heat from?
P. Is it from the sun?
M. It is. Since the rays of the sun warm whatever
they fall upon, they also form a kind of energy which
is called light, or radiant energy. The sun gives the
work which evaporates the water, and lifts the vapour
into the air. When the water falls again as rain or
snow, the work is partially restored; for example, it can
drive a mill.
P. So mills are really driven by the sun?
M. Yes, for if it stopped shining all streams would
stop running. Besides, windmills are driven by the
sun, for wind is the result of its action.
P. How it all hangs together! 1 look upon the sun
and the rain with quite different eyes now.
M. You will learn many more such connections.
STEAM, l8i
Now let us get back to the property which water has of
dissolving substances. When water has dissolved any
substance, it is said to form a solution. Such solutions
are much more in use than the substances themselves.
P. Why?
M. Because of their chemical action. Solid sub-
stances do not act at all on each other, or only very
slowly and incompletely. In order that they may act
upon each other chemically they must be brought together
in the Hcjuid state. This can happen in one of two ways;
they may be melted or they may be dissolved. Melting
requires a high temperature, as a rule, which is not easily
attained, whereas it is easy to make a solution. More-
over, many suljstances do not stand a high temperature
without changing.
P, I begin to see that water is almost the most impor-
tant thing in the whole of chemistry.
M. Not alone in chemistry, but also in daily lifei All
food contains more or less water; tea, coffee, milk, wine,
beer, are solutions and to some extent mechanical mix-
tures of various substances in water; blood and all other
juices of the body are also aqueous solutions. So is the
sap of plants; you know that every plant dies when it is
dried, that is, when water is removed from it. And
that is also the case with animals.
P. I shouldn't have dreamt that water was such an
important substance; you might even say. No water, no
life!
M. Of course you can also say: No oxygen, no life; no
nitrogen, no life; no iron, no life, and so on. Life is
such a very complicated affair that many conditions
must be fulfilled before it occurs. You can picture it
as a stretched chain consisting of different kinds of links;
1 82 CONVERSATIONS ON CHEMISTRY,
when one of the links is broken, the chain breaks, no
matter how strong the other links are. In the same way
life stops if any one of the necessary factors is wanting,
so that none of them can be called the most important.
22. NITROGEN.
M. To-day we shall learn something more about air.
P. We are taking up all the elements one after
another; first fire, then water and earth, and now air.
M. The old Greeks called them elements because they
were universal and their importance could not be over-
looked. And as we too are considering the most impor-
tant things we naturally come to them. What do you
know about air?
P. That it is a gas, but not an element ; it is a mixture
of one-fifth of oxygen and four-fifths of another gas —
M. Which is called nitrogen. I told you, too, that
nitrogen, like oxygen, has neither color, odor, nor taste,
and that it differs from oxygen in not supporting com-
bustion. It is not combustible, and so differs from hy-
drogen.
P. Then does nitrogen combine neither with oxygen
nor with other substances ?
M. Not as a rule; nitrogen is an unsociable character —
it likes solitude, doesn't care to unite wdth other elemen's,
and, even after it has united, it separates as soon as it can.
That is the reason why air consists mostly of uncombined
nitrogen. For as it is a gas there is no other place for it.
P. Doesn't it dissolve in water?
M. Very little, even less than oxygen. We will make
some nitrogen. How can that be done?
P. We require only to separate nitrogen from the air.
NITROGEN.
183
M. Quite right; how shall we do that?
P. Oh, I suppose by burning something in the air;
a candle?
M. That has several disadvantages. First of all, other
gases are produced at the same time which remain mixed
with the nitrogen ; and second, a candle goes out long before
the whole of the oxygen has been removed. Here is
another means of removing oxygen: it is phosphorus
(see page 105). It has the property of using up all oxygen
even at ordinary temperature. I place in a test-tube a
piece of phosphorus which has been made fast to a wire by
melting. Then I invert the test-tube over water (Fig. 38).
You see that a white vapor flows down from the phos-
Fig. 38.
phorus; it consists of the products of oxidation and con-
tains oxygen. At the same time the water begins to ri^e
slowly, and after about an hour the cloud is no longer
visible, showing that all oxygen has been used; a fifth of
the air has disappeared. I have here a flask in which
phosphorus has been kept since yesterday. It
tains only nitrogen.
P. It looks exactly like air.
now con-
1 84 CONVERSATIONS ON CHEMISTRY.
M. You will soon see that it is not. I dip in a burning
splinter, and it goes out just as if it were plunged into water.
P. Give me some phosphorus; I want to repeat that
experiment.
M. I would rather not, for phosphorus catches fire
very easily, and, moreover, is very poisonous. I will
tell you another plan. There is a compound of iron, a
sulphate; it is a green salt. If you dissolve it in water
and mix the solution with lime you get a thin paste which
takes up oxygen very quickly. I will make such a paste
in this large flask, and, after I have corked it, I shake
it thoroughly. If I place its neck under water and take
out the cork, water enters — a sign that some of the air has
disappeared.
P. Let me try with this splinter. Quite right. It has
gone out.
M. There is not much more to show you with nitrogen,
as it does not combine with any common substances.
P. Is it light, Hke hydrogen?
M. No; since it is the chief constituent of the air it
has about the same density as air. It -is lighter than
air because oxygen is a little heavier.
P. Then nitrogen appears to be a sort of indifferent
element, which is unnecessary for changes on the earth.
M. No, that is by no means the case. Nitrogen is
equally important in peace and in war, first because it
is a constant constituent of all living creatures, animals
as well as plants; and second because compounds of
nitrogen form gunpowder, artificial dyes, and innumer-
able other substances which are equally important for
industry and daily life. While free nitrogen costs nothing,
because you can have as much as you like of it in the
air, combined nitrogen has a fairly high value; it costs
about lo cents a pound.
NITROGEN, 185
P. Why don't they make compounds using the nitro-
gen of the air?
M. You will find a serious difficulty there. Making
the free nitrogen of the air combine with other sub-
stances is such an expensive operation that the price of
the compound is prohibitive.
P. How can that be? It costs nothing to change
oxygen or hydrogen into compounds; the change happens
by itself.
M. There is the difference. With nitrogen it doesn't
happen by itself. I see you ask why not. The answer is
that hydrogen and oxygen when they enter into combina-
tion give out energy ; indeed, you saw how much heat is pro-
duced by their combination. But to make nitrogen combine,
work or energy must be spent. And since work is never a
free gift, combined nitrogen has a much higher value than
free nitrogen, although it is the opposite with hydrogen.
P. But not with oxygen?
M. Plants make free oxygen; we shall come to that
later. And because free oxygen can't exist in plants,
but spreads itself through the air, it costs nothing. If
oxygen were a solid or a liquid substance it would be
collected just as we gather grain and fruit, and would
be sold.
P. So the value of these substances does not lie in
themselves, but in the work which is connected with them.
M. The thought is right, but you haven't expressed it
correctly. Different substances do not exist without
carrying with them the corresponding amounts of work
or energy; therefore you cannot talk of them without
speaking of this energy. The fact is this: in certain
cases the uncombined elements contain more energy
than their compounds; in other cases, as with nitrogen,
the opposite is the case. According as one or the other
1 86 CON VERS ATlOm ON CHEMISTRY.
relation prevails, the elements or the compounds have
the higher value.
P. But their value consists really in their energy.
M. Yes, that is right on the whole.
P. How does it happen that compounds of nitrogen
are important for war, as you said, because gunpowder
is made from them? Has that anything to do with
the question of work?
M. Of course. A gun is also a machine for doing work.
P. No, indeed! It is used for destruction and not for
worL
M. What you call destruction is also work. The
object is to give to the ball a certain high velocity, and
in order to do that, as you know from throwing stones,
a good deal of work must be expended. ♦
P. Yes, now I understand. With gas-engines, of
which you spoke to me before, an explosion is also used
to make work.
M. Quite right. And if large blocks of stone or of ice
have to be got rid of (and that involves a great deal of
work), they are blown up with powder, as you know.
There you have the work done with the powder before
your eyes.
P. Yes, I see that. But what has it all to do with
nitrogen ?
M. Well, in compounds of nitrogen there is more
work than in free nitrogen, and so these compounds can
be used to do work.
P, Oh, that is the reason, is it?
M. Yes; at least a partial reason.
P. Please answer this question that I wanted to ask
before. You said that nitrogen was so easily produced
from its compounds. How does it happen that there
is any combined nitrogen, and that it is not all free?
NITROGEN. 187
M, That is a very good question. The answer is
that by many kinds of work which are available in
nature, free nitrogen is brought into combination. For
example, many plants, like peas, beans, lupins, and
vetches, have the property of using part of their work
in causing nitrogen to combine. When an electrical
discharge, which you call Hghtning, passes through the
air, nitrogen also enters into combination. Besides that,
people are very careful not to waste combined nitrogen.
The dung of animals contains a large quantity, and the
farmer spreads it on his fields, where it is absorbed by
plants.
P. So that is why they spread it on fields. I could
never see why that nasty-smelling stuff could do any good
to plants.
M. Besides containing combined nitrogen, manure con-
tains other substances which plants require, but nitrogen
is the most important because it is the dearest. More-
over, though manure could be made to have no smell,
it would not help us, because the evil- smelling substances
contain nitrogen, which would be lost if they evaporated.
P. So the bad smells come from nitrogen?
M. A good many do. Do you know the smell that
wool gives off in burning?
P. Yes; it is abominable.
M. Many other substances give a similar smell; for
example, horn, flesh, leather, and feathers. All these
substances contain nitrogen, and that forms a means of
recognizing them. Wood and sugar and starch also give
disagreeable smells when they burn, but they haven't
this extremely unpleasant odor; they contain no nitrogen.
P. When milk boils over in the pan it smells as un-
pleasant as burnt hair. Does it contain nitrogen too ?
1 88 CONyERSATIONS ON CHEMISTRY.
M. Certainly; casein, which is contained in milk, is
a compound of nitrogen.
P. Is casein contained in cheese?
M. Yes.
P. Old cheese has a different kind of smell.
M. That is also due to its containing compounds of
nitrogen.
P. Do all nitrogen compounds smell bad ?
M. Not all, but most of them. But nitrogen is not
the only element that has this unpleasant property.
Many sulphur compounds have a disagreeable smell,
though of quite a different character.
23. AIR.
P. You told me yesterday a great deal about the com-
pounds of nitrogen, but you didn't show me a single one.
There must be hundreds of them.
M. That is quite true. You will have to wait till
later to learn about the individual compounds, because
they exhibit pretty complicated relationships. For the
present we have still much to learn about free nitrogen.
P. I thought there wasn't much to say about it; you
said something of that sort.
M. Yes, so far as concerns its properties as an element.
But because nitrogen is the chief constituent of the air,
we must take air as our subject. Our whole life and
everything that we do takes place in air, and so we must
learn about its properties and know how to interpret them
rightly, in order that we may not make frequent mistakes.
P. Yes, no one can live without air. But you told me it
was the oxygen which was necessary to life, and that
animals are suffocated when placed in nitrogen.
M. Quite right; but we will not discuss that again.
AIR.
189
Air, however, is a gas; it is the best known and most
widely spread of all gases. For that reason we will
now study the properties of gases, taking it as an example.
P. I'm glad of that, for I must say that gases always
strike me as queer. It is easy to see and grasp solid and
liquid things; but whether a flask contains hydrogen, or
oxygen, or ordinary air, I'm sure I can't tell. For all I
know there might be nothing in the flask.
M. Yes, I quite beheve you; for as gases are hardly
ever visible, people don't generally know much about
them. So I will show you something. You know that
we live surrounded by gas, by air. A wind or a storm will
teach you that air is a substance; just as a soHd or liquid
body in motion can move, throw down, and break other
bodies, so also can air in motion.
P. Why can't we see air?
M. Because we are surrounded by it. Fishes can't
see the water in which they swim. But when air is
surrounded with water it becomes
visible. I blow air through this
tube into a tall glass full of water.
Now you can see little quantities
of air, as round bubbles, quite well
(Fig. 39).
P. But I see nothing in the
bubbles themselves.
M. Of course not, because air is
transparent. You see nothing in
the water in the glass; you only
recognize the surface which divides the water from the air
in the glass. It is exactly the same with the air-bubble.
P. But I can't understand in the least how you can
see air in water when they are both transparent.
Fig. 39.
190
CONVERSATIONS ON CHEMISTRY.
M. It is true they are both transparent, but they affect
the passage of Hght in different ways. In physics we call
that the difference of rejractivity. For that reason, too,
you see no particular colour, but only differences between
light and dark. — But now we will consider the air from
another point of view. In your lessons in physics you
have learnt something about the pressure of the atmos-
phere and about the barometer; let us take up that
now. What is a barometer?
P. A tube full of mercury, closed at the top and open
below.
M, Yes, that is pretty correct. I have here a glass tube
which can be closed above by a glass stop-cock. At the
« bottom is a narrow rubber tube, attached
7^ by its other end to a second tube which is
open (Fig. 40). I open the stop-cock and
pour mercury into the other tube, till the
rubber tube is quite full and the glass
tubes are half filled with mercury. I fix
each tube vertically in a retort- stand;
now, what is the level of the mercury ?
P. It should stand at the same height
in both legs, and so it does.
M. And now, when I raise the plain tube ?
P. The mercury will rise on the other
side too. Take care; it's running through
the stop-cock.
M. I will shut it, then. Now I will
lower the other tube again. But the
mercury doesn't follow down; it remains
close up to the top. Why?
P. Because the top is shut. No ail
Fig. 40. can enter.
/ilR, 191
M. What has air to do with the level of the mercury ?
P. I suppose it has something to do with the pressure
of the atmosphere. Wait, let me think. Yes, air can
press on the mercury in the open tube, but not in the
closed one.
M. Quite right. But now the mercury begins to
sink below the stop-cock. That isn't because the top
leaks, for when I raise the other tube the mercury rises
again to the stop-cock; and when I lower it the mercury
falls again. What is the reason of that?
P. The pressure of the air can't keep the mercury
up any longer.
M. That is so. If I raise the open tube the mercury
rises in the other one, and when I lower it, it falls. Now
we will take some measurements. I place both tubes
close together and measure how much higher one column
is than the other. It is 75 centimeters. If I now move
the plain tube up or down the difference is always 75
centimeters. The pressure of the atmosphere is there-
fore 75 centimeters of mercury.
P. Yes, that is the height of the barometer. Is that
the same as 30 inches?
M. Yes, 30 inches is nearly equal to 76 centimeters.
But the pressure of the atmosphere is a pressure, and
75 centimeters is a length. How can you express a
pressure in units of length?
P. The pressure of a liquid depends on its height.
M. Doesn't it depend on the width of the column of
liquid ?
P. No, I learned that it depends only on its height.
M. Yes, for any one liquid. But for different liquids
the pressure depends on the density too. Mercury is
13I times as heavy as water, and so for an equal height
192 CONVERSATIONS ON CHEMISTRY.
it presses 13 J times as strongly. If you wish to get the
same pressure with water as with mercury —
P. The height should be 13 J times less.
M. That's the wrong way round. Think in words.
P. Mercury is 13 J times as heavy as water, and so it
presses 13 J times stronger, or water presses 13 J times
less than mercury — yes, now I see, the column of water
must be 13 J times the height of the mercury.
M. That is right now. What would be the height of
a water barometer then?
P. 13 J times 75 centimeters is 1012J.
M. Yes, more than 10 meters. Now can you remember
whether the pressure of the atmosphere always remains
the same?
P. No, it changes; in fine weather the barometer
stands high, and when it rains it stands low.
M. Yes, a high pressure often means fine weather,
and vice versd; but this isn't always the case, because the
atmospheric pressure is affected by many different con-
ditions; however, we won't discuss that just now. You
know that the normal height of the barometer is 30 inches,
which is practically the same as 76 cms., and that
pressure is used as a standard and is called an atmosphere.
Can you give the meaning of the word atmosphere?
P. Yes, air.
M. The derivation of the word means a sphere or
ball of air. It refers to the pressure of the atmosphere.
In physics the pressure is usually measured in centimeters
of mercury. Now pay attention to this : i atm. = 76 cms.
mercury; and i cm. mercury = ^J^^- atm. But to-day the
pressure of the atmosphere is only 75 cms., that is ^f,
or 0.987 atm. When I open the tap and let air in after
I have raised the other tube, I take in a definite quantity
AIR. 193
of air, and I know that, like all the air in this room, it
exerts a pressure of 75 cms. of rhercury. I level the
mercury so that it stands exactly at the mark 100. That
means that the tube contains exactly 100 cubic centi-
meters of air. Now I close the tap again, so that this
quantity of air can alter its volume only when the level
of the mercury is altered. Now the apparatus is ready
for the experiment.
P. What are you going to do?
M. I wish to show you how the volume of the air
changes when the pressure is changed. First I will
lower the other tube; what do you see?
P. The mercury in the tube with the stop cock falls,
but much less.
M. Now we will measure the volume of the air, and
the pressure. I can read off the volume from the division
on the tube; it is 120 cc. To find the pressure I must
measure the distance between the two mercury columns;
it is 12.5 cms. What is the pressure of the air now?
P. 12.5 cms. of mercury.
M. Wrong.
P. You said it yourself.
M. I said that the difference was 12.5 cms. In which
tube does the mercury stand higher?
P. In the tube with the tap where the air is contained.
Yes, the pressure must be less there.
M. Less than what?
P. Than it was to begin with.
M. Quite right. What was it to begin with?
P. I don't know.
M. Yes, you do. Just think! What did I tell you at
the beginning of the experiment ? What was the pressure
of the air when I turned the stop cock?
1 94 CON VERSA TIONS ON CHE MIS TR Y,
P. Oh, now I remember, it was the pressure of the
atmosphere, 75' cms.
M. And what is it now?
P. 12.5 cms. less, that is 62.5 cms. Is that right now?
M. Yes. Let us set it several times and measure the
volume and the pressure of the air. We will make a
tabk.
Pressure,
Volume.
75 cms.
mercury
100 CCS.
62.5 "
(<
120 "
60
((
150 "
37-5 "
((
200 "
25 "
((
300 ♦'
P. What is the use of that?
M. I want to show you how to discover a law of nature.
We have two quantities, pressure and volume, which
change with each other; whenever we give one some
definite value, the other must also have a definite value
which we cannot control.
P. But the volume depends only on the pressure; I
don't see how the pressure depends on the volume. To
get a definite volume we must alter the pressure.
M. That depends on the apparatus we are using. If
you were closing the mouth of an empty bicycle pump — •
I mean one filled with air — and then press in the piston,
you alter the volume of the air, and you can easily feel
that the pressure rises, because it is more diiB&cult to push
in the piston.
P. Yes, I see that.
M. Well, then, you see from the table that the greater
the pressure the smaller the volume. If we call the
pressure p and the volume v we know that for each value
of p there is a definite value of v.
P. What is the law for this?
AIR. 195
M. It should make it possible to calculate for each p
the corresponding v, and vice versd.
P. How can that be done?
M. By finding a method of calculating, or, as it is
called, a formula, by means of which the one value can
be calculated from the other.
P. I don't understand.
M. Suppose you have 10 apples; some in your pocket,
the others in your hand. If we call / the number of
apples in your pocket and h those in your hand, if you
know h you can calculate /, and if you know t you can
calculate h. How is this possible?
P. Because I know that the sum of both is 10.
M, So the sum of t and h is equal to 10, and the formula
is
t+h= 10,
From this formula you can calculate ^ if /t is given, or h if
you know /.
P. That is very nicely put. But when I come to think
of it it is quite unnecessary, because I know it without
the formula.
if. You only think so because the formula is so simple
and the process so common. But perhaps we can bring
our measurements of the pressure and volume of the
air into a similar simple formula.
P. Let me try. 75+100=157, 62.5+120=182.5,
60+150 = 210. No, that won't do, the sum always gets
bigger.
M. So the addition formula doesn't work. You might
have seen that at first. For you can only add similar
quantities, like apples to apples, but not different quan-
tities like pressures and volumes.
196 CONVERSATIONS ON CHEMISTRY,
P. What sort of formula can it possibly be?
M. If p grows larger, v grows smaller. What com-
bination of p and V has this property?
P. A great many, no doubt.
M. Of course, but not many simple ones. Try to
find the simplest possible one.
P. Perhaps the product ? If one factor becomes larger
the other must grow smaller to get the same product.
M. Try if that works.
P. 75X100=7500, 62.5X120 = 7500, 50X150 = 7500,
37.5X200=7500, 25X300=7500. Upon my word it's
right!
M. Then write the formula.
P. pXv^JS^'
M. Right. Now you have found the law which con-
nects the pressure and volume of the air with each other,
or makes them dependent upon each other.
P. I should never have found that out without your
help.
M. I quite agree.
P. Tell me, did you find it out by yourself ?
M. No. An English physicist named Boyle dis-
covered it nearly two hundred and fifty years ago, and
it goes by the name of Boyle's law. But we haven't
quite grasped the law yet. Supposing the pressures had
not been given in centimeters of mercury, but in atmos-
pheres, all the values of p would have been 76 times less.
Then the product py.'v would not have been equal to
7500, but '^^776 = 98.7? and the formula would have been
P. I see that.
M. Further, if I had not had 100 cc. of air, but only
80—
^IR. 197
P. Then the product would have been 75X80 = 6000.
M. Yes, the first one would be. But what about the
others ?
P. That can't be told beforehand.
M. Oh, yes, it could; only think. I had taken ^Yioo
or Ys of the original quantity or air. Whatever I do
with the air, this quantity always remains Yg of the
original. And therefore its volume remains under all
circumstances Ys of its original volume. Hence all the
figures for v would be decreased in the same proportion.
P. Wouldn't the values of p be also proportionately
decreased ?
M. No. The pressure is equally distributed through
the whole quantity of air, and so it doesn't matter whether
you take a larger or a smaller fraction of it. The 100 c.c.
with which the experiment was made were, of course,
only an arbitrary amount of the whole of the air in the
room, which had everywhere the pressure of 75 cms.
P. Why do the pressures behave differently from the
volumes ?
M. As I have often said to you in such cases you
mustn't ask why, but notice that certain quantities behave
in one way and others in another. Temperatures be-
have like pressures. For instance, if a mass of water
has a definite temperature every part of the mass has
the same temperature whether it is large or small.
P. But surely a quantity of water can have different
temperatures at different places
M. Quite true, but I was speaking of masses which
have the same temperature all through. You see the
similarity here between temperature and pressure. If
they have different values at different parts of a continu-
ous mass they don't remain in that condition, but become
ipS CONVERSATIONS ON CHEMISTRY.
equal. But we must go back to our experiments. You
have seen there is something arbitrary about the number
7500, since it depends on the quantity of air taken and
on the units in which the pressure was measured. We
must give our formula such a form as not to contain
any arbitrary units. Hence we write Boyle's law thus;
pv=C.
P. What does C mean?
M. It means that the product pv has a definite value
C, which remains unchanged as long as only the values
of p and V change. For this reason p and v are called
variables, while C is a constant; that is, an invariable, or
an unchangeable number.
P. But C can have different values too.
M. Only when you change the quantity of air taken.
You have already seen that the product pv increases
or diminishes its value in proportion to the quantity
taken. If you call q the quantity, you can write C = qKy
where K is another constant which is no longer dependent
on the quantity q. Place this value of C in the equation
and it becomes
iru
pv=qK or —=K,
P. What's the use of this formula?
M. It makes it possible to apply the law to any quan-
tities of gas. If the amount in cubic centimeters is
measured at 75 cm. pressure, our former constant C = 7500
would be written: 75oo = iooi<C or K=yS' If the num-
ber 75 be introduced into the last equation, then
pv
AIR. 199
And this equation holds for all experiments with any
quantity of air you like.
P. I should like to see that.
M. We will make an experiment. I shut off 60 c.c.
of air, at atmospheric pressure, lower the other tube till
the volume of the air is 100 c.c. What is the pressure
now?
P. How can I tell?
M. You ought to be able to tell; it follows from the
formula. You have only to introduce the values in
order to calculate p. You know the volume v=ioo and
the quantity q = 6o.
_. ^Xioo ^ .
F. — T = 75j so /> = 45; the pressure is 45 cms.
M. Now, how do I get the pressure 45 cms.?
P. Stop; I can calculate that. The pressure of the
atmosphere is 75 cms., and 75—45 = 30, so the mercury
in the open tube must stand 30 cms. below its level in
the other. Let me measure it. It agrees exactly.
M. Are you surprised?
P. Yes; it is almost like magic.
M. What is?
P. That you can tell such a thing beforehand.
M. That is the use of laws of nature to enable us to
foretell what will happen in the future. Only think of
the predictions of the eclipses of the sun and moon.
P. Yes, IVe taken it all in, but I can't say that I'm
accustomed to it yet.
M. That is quite to be expected; but we shall often
have to do with similar things, and you will soon become
famihar with such ideas.
200 COI^yERSATIONS ON CHEMISTRY,
24. CONTINUITY AND EXACTNESS.
M. Have you understood all I told you about Boyle's
law?
P. Yes, I understood all that you said. But I couldn't
understand a great deal of what you didn't say.
M. Well, then, ask questions.
P. Yesterday we found the pressures corresponding to
five or six different volumes. And you got out the
formula ^=7500, which fitted these few cases, and
then you used them for quite different cases. How did
you do that?
M. It's a very sensible question, and I will try to make
it clear to you. When you blow several times into a
toy trumpet, you always hear the same tone; you will
expect always to hear the same tone in future whenever
you blow the trumpet.
P. Of course.
M, It is much the same with the formula. When-
ever you multiplied pressure and volume together the
product was 7500, and hence I had the right to expect
that it would always be the same. You may remember
that our expectation was not disappointed. We tested
the formula, and found it held.
P. Ohl I didn't think it was as simple as that.
M. Nor is it. It is an instance of a very important
law, one which is so general and accepted that we always
use it.
P. A general law! I don't know of any in this case.
M. Certainly you do, because you constantly use it.
The fact is that you are not in the habit of expressing
CONTINUITY AND EXACTNESS. 20I
It in the form of a law. It is the law of persistence
oj natural phenomena.
P. How is the law expressed, then?
M. If an event takes place under definite conditions it
will always take place in future ij the conditions are the
same.
P. But that goes without saying.
M. People always say that of things they haven't
thought about. You had just put a question which was
answered by the law.
P. Yes, but that was a case that was new to me.
M. It was only a new application of the general law;
it was no new law. You see at once how very import-
ant it is to express definitely such self-evident laws. If
you had known how to express this law before, you could
have answered your own question.
P. So I will in future. — But what we have just been
talking about wasn't all that I wanted to know. Of
course, I believe that if we were repeating exactly the
same experiment with the same volumes, we should get
the same pressures. But there are many other pressures
and volumes between those that we didn't measure-
How can we know that the formula will suit these, for
the conditions are no longer the same ?
M. That is a very good question. It is also answered
by a law of nature.
P. Another!
M. You are getting tired of laws of nature, are'nt
you? Don't be frightened; it is another self-evident
one.
P. All that I mean is that we will soon have so many
that there won't be another left to be discovered.
M. So much the better.
202 CONVERSATIONS ON CHEMISTRY.
P. So much the better?
M. Laws of nature tell us what we have to ex-
pect under definite conditions. Now no law includes
all such conditions, but only one or a few. Hence,
in order to know exactly what will be the final
result, we must know the laws for all possible
conditions, so that all uncertainty disappears and
only one thing is possible. Then, that particular result
will really take place.
P. Oh ! so what you really meant was that there should
be no doubt of the result.
M. That wasn't quite your view, was it? The general
law of which I spoke is one that relates to the continuity
of natural phenomena.
P. Explain that.
M. We have just seen that we can express natural
laws in the form : — if this is the case the other will follow.
Now the "this" is often not one single definite thing,
but something capable of different degrees, shades, and
magnitudes, and other things depending on these. If
we alter one of these continuously, that means, so that
there is never a sudden change in its value, the other also
alters continuously, and its value makes no sudden
change, either.
P. I think that I can remember a Latin proverb
about that. Natura non jacit saltus Nature makes
no jumps.
M. Yes, but it's only a half-truth. Nature does
make jumps; but then all magnitudes which are
connected with each other make jumps at the same
time.
P. I can't think of an example.
M. Just think of the transition from ice to water.
CONTINUITY AND EXACTNESS. 203
When the solid changes to the Hquid its physical state
changes suddenly, and at the same time its volume sud-
denly becomes one- eleventh smaller, and its pow^er of
refracting light, its electrical properties, and countless
other things suddenly change their value too.
P. Do all its properties change at the same time?
M. Nearly all; mass and v^eight, however, remain un-
changed.
P. I still don't see vv^hat all this has to do with my
question.
M. You asked how it was possible from a few values
of pressure and volume* for which you found the product
constant, to assume all intermediate values to be the
same. But it can be deduced from the law of continuity.
For if the product is the same for any two values of
pressure which lie somewhat near each other, then it
must also be the same for the pressures which lie between
these two, unless one of the factors suddenly alters its
value. But the law of continuity excludes such a pos-
sibihty.
P. I don't quite understand that.
M. Think of the example of the toy trumpet. If you
blow gently the first time and hard the second, and the
tone is always the same height, you might conclude that
if you were to blow only fairly hard, you would still hear
the same tone.
P. Yes, of course.
M. You have just appHed the law of continuity.
P. Oh, is it as simple as all that?
M. It is; the difficulty isn't in understanding the law,
but in applying it to unfamiHar cases. But now we will
go on with Boyle's law. Up to the present we have only
tested it for pressures which lie below that of the atmos-
204 CONVERSATIONS ON CHEMISTRY.
phere. What do you think — will it hold for higher pres-
sures ?
P. I don't know any reason either for or against.
M. Yes, you know one in favoui of it: the law of
continuity. Try to apply it.
P. At pressures which are a little higher than one
atmosphere the product pv will still be the same.
M. Quite right.
P. But how far can I go?
M. You can only tell by experiment. We will lift up
the open tube as far as we can. Now the volume has
contracted to 40 c.c, and the difference of the mercury
level is more than one metre. I will use a second metre
rule; now I find 112 J cms. Does that agree?
P. 112^X40 = 4500. No, the product is much too
small.
M. Now just think.
P. Of course I forgot the pressure of the atmosphere.
But I can't subtract 112 J from 75
M. Why should you?
P. Because — no, how stupid I was; the mercury is now
pressing on thfe same side as the atmosphere, so I must
add them and not subtract: ii2j+75 = 187J, and
1871X40 = 7500. It's quite correct.
M. What conclusion would you draw for pressures
which lie between that and one atmosphere?
P. The product will agree for those too, according
to the law of continuity.
M. You needn't laugh, it's quite right. To convince
yourself you may make some measurements with the ap-
paratus.
P. That's capital; thank you very much.
M. Only take care that you don't spill any mercur)^;
CONTINUITY /IND EXACTNESS, 205
you had better take the lid of a large pasteboard box and
work with your apparatus on it.
M. Well, have your measurements been successful?
P. Not very. The product of pressure and volume
didn't always come to 7500; it was sometimes a little
more, and sometimes a little less.
M. That is what you might have expected.
P. Then is Boyle's law not exact ?
M. The law is, but not your measurements. How
accurately did you read the levels ?
P. Oh, it wasn't easy to hold the rule upright and to
read the level of the mercury.
M. Look here^ you can't have been a whole centi-
metre wrong, though you may have made a mistake of
some millimetres. Look at my last result, where the
volume was 40 c.c. and the pressure 187J cms. If I
had read J a cm. too much (which was quite possible
because I had to lengthen the rule) I should have got
188X40 = 7520 instead of 7500. If I had read J cm.
too little the product could have, been 7480. That shows
the influence of experimental error on the result.
P. Yes, my numbers were something like these.
M. Moreover, you may have made a mistake in
measuring your volume. The tube is graduated in
cubic centimetres and tenths, and you may have made
the mistake of a tenth. If you had read 40.1 instead of
40 it would have given 1871X40.1 = 7518.75, again an
erroneous result. If you had also read the pressure
wrongly, say 188 cms., the product would have been
7538-8.
2o6 CONIFERS A TIONS ON CHEMISTRY.
P. Well, how can you know which is the right
number ?
M. You can never know, for every experiment con-
tains an error.
P. Even when you measure very accurately ?
M. Then you will make your error smaller, but it will
never disappear.
P. But is nothing ever measured accurately?
M. No magnitude is ever measured so accurately that
there is no error at all. There are only measurements
of greater or lesser accuracy.
P. But what can be done if the numbers are as differ-
ent as those I got? What value should be taken as the
correct one?
M. You can't state the correct value, but you
can state one which is probably the nearest to the cor-
rect one.
P. How can that be done?
M. Think for a minute. You may have made errors
which would have made the result too large, as well a
too small, and so the correct value will lie somewhere in
the middle, between the largest and the smallest values
which you found.
P. I see that.
M. Then you must take the mean value of all your
observations. That can be done by adding all the
observed values together, and dividing by the number
of observations. The quotient is called the mean value,
and it is the most probable one.
P. Please let me try so that I may understand it.
I found for the product pv the numbers 7520, 7475,
7492, 7533> 7506, 7491-
M. There are six values; add them and divide by six.
CONTINUITY AND EXACTNESS. 207
P. 7520
7475
7492 .
7533
7506
7491
45017; =7502.833. . . . How many decimals
shall I write?
M. None at all.
P. But then I shall make a mistake.
M'. You know that all your measurements con-
tain an error. If you examine your numbers you will
see that even the tens don't agree, and the units must
be quite uncertain. Of your mean value 7502.833 . . .
the o in the tens place is perhaps right, but the
two units are quite uncertain, because they would have
come out differently if you had made another measure-
ment.
P. So I will. — It comes out 751 1.
M, Do the calculation with the seven values now;
what do you find?
^ 52528
P' '-^ = 7504.
M. You see you have got two more units. It
would lead to error if you were to write units or deci-
mals. The usual plan is simply to write o for such
uncertain places to show that you can make no statement
about them. Do that ; what is your mean value ?
P. 7500.
M. Quite right. — Now let us go back to the question
whether Boyle's law holds for any pressure. The
2o8 CONyERSATIONS ON CHEMISTRY.
answer is that it has been shown to be nearly correct
for the smallest pressures which can be measured. On
the other hand, at higher pressures there are deviations,
small indeed at lo atmospheres, considerable at loo,
and very great at looo.
P. Where do these deviations begin?
M. The answer depends upon the exactness of the
measurements. The more exactly the pressures and
volumes are measured, the smaller are the pressures at
which the first deviations can be detected.
P. So Boyle's law is not exact.
M. No; that can't be said of any law of nature.
But so far as its apphcation is concerned, it is exact
enough, because the errors of our measurements will
always be much greater than the deviations from the
law.
25. THE EXPANSION OF AIR BY HEAT.
M, Do you quite understand Boyle's law now?
P. I think I do, but there is something I am not clear
about. You once told me that air expands when it is
heated. But then the same quantity of air at the same
pressure would have different volumes; it would have a
bigger one if it was warm and a smaller one if it was
cold.
M. You are quite right. Boyle's law holds only for
an unchanging temperature.
P. What temperature?
M. For any temperature, but it must remain constant.
Our experiments were carried out at the temperature of
THE EXPANSION OF AIR BY HEAT. 209
the room, which was about 18°. If it had changed
much while we were making them our results would not
have been concordant.
P. Does that mean that Boyle's law is not worth much ?
M. It has lost none of its value, only you have learnt
one of the conditions which must be fulfilled when it is
appHed.
P. If the temperature doesn't remain the same, what
can we do?
M. Then we try to find out a law which will allow
for its influence.
P. How can that be done?
M. If we know how much the volume of a given quantity
of gas alters when the temperature is changed by a known
amount, we can apply a correction to our measurements,
so that the result comes out as if it had been obtained
at some one definite temperature.
Pj I have a sort of general idea, but am not clear about
it yet.
M. You will soon understand it. Here is a narrow
glass tube about 2 mms. wide, about half a metre long,
closed at one end, and containing at about the middle a
drop of mercury, which shuts off a definite quantity of
air. If I warm this air with my hand, the drop moves
forward, and it goes back again when the air cools.
Thus you can see and measure the expansion of the air
by heat.
P. It is just like a thermometer.
M. Yes, an air-thermometer. Now I place the tube
in melted ice and mark where the drop stands with a
small india-rubber ring.
P. Where did you get it?
M. I cut it off a piece of rubber tube with a pair A)f
2IO
CONVERSATIONS ON CHEMISTRY.
scissors. Now I measure the length of the column of
air which is cooled to o° in the
ice; its length is 273 mms.
Now I will heat the same quan-
tity of air to 100°, the boihng-
point of water. To do this I
fit a flask with a somewhat wide
glass tube by means of a cork,
and boil the water in the flask
(Fig. 41). When I place the tube
in the steam the drop moves up
considerably.
P. How can you make a mark
on it without burning your fingers ?
M. I push down the second
india-rubber ring with a rod to
the right place. Now it is done.
I take the tube out and measure
the length of the column of air.
The second ring stands at 373
mms.
P. That is exactly 100 mms. more; a millimetre for
each degree. How does that come out so exactly?
M. I knew beforehand that 273 volumes of air when
heated from the freezing- to the boiling-point of water
would expand exactly 100 volumes, and I enclosed exactly
the right quantity of air to begin with.
P. Did you do that at 0° or 100° ?
M. No; I looked at the thermometer in the room and
saw that it stood at 18°. As 273 divisions at 0° expand,
one division for each degree, I knew they would occupy
at 18° 273-1-18 = 291 divisions. So that I placed the
drop of mercury 291 mms. from the end of the tube.
Fig. 41.
THE EXPANSION OF AIR BY HEAT. 211
P. How did you do that? The drop doesn't move
when I tilt the tube.
M. That is very simple; it doesn't move because the
air can't pass the drop. I pushed a horsehair into the
tube through the drop and it moves quite easily now.
Look!
P. That's neat. But how did the air pass? Oh, I
see; the mercury doesn't quite touch the glass where the,
hair is.
M. Yes; what is called surface-tension makes the mer-
cury curved, and the air can not pass except through the
narrow groove between the glass and the hair. But we
will go back to our experiment. We will make a
diagram (Fig. 42). The horizontal line stands for a
thermometer. At the mark o water freezes, at 100 it
boils.* Each millimetre stands for one degree.
P. I understand that.
M. Now we will draw horizontal lines each of which rep-
resents the volume of the air in our experiment. We will
make the horizontal line at o, 273 mms. long; and at 100;
373 mms. We join the two end-points by a straight line.
P. What is the use of the figure?
M. It makes it possible to find the volume of the air
for any intermediate temperature. Pick out the point
18° on the thermometer-line and measure how long the
horizontal line is.
P. It is 290 — no, 291 mms. long. I have just had this
number — yes, it was the place where you set the mercury
with the horsehair.
M. Yes, that is the volume of the air at 18°.
P. How does it happen that the right number comes?
* Fig. 42 is reduced to a quarter of its right size.
212
CONVERSATIONS ON CHEMISTRY,
M. That is very simple; for each degree the length of
the air-line increases by i mm., and so the ends of all
these Hnes lie in a straight line.
P. Yes, I understand that. I see, too, that I can find
-
""■
4'JO
—
—
—
—
/
/
/
/
,
)0-
-
-
-
-
-
-
/
/
/
-
-
-
/
/
/
,
/
V
/
/
/'
^
50
/
/
/
-
y
/
K)-
-
-
-
_
-
-
/
/
-
-
-
/
/
/
/
JU
/
/
/
/
/
/
/
/
/
/
6
0
/
/
/
/
-2
rs
-2
50
-
-
'2<
30
-\l
)0
-IC
K)
-S
0
{
)
s
0
1(
X)
Fig. 42.
out what the volume of the air v^ill become when it has
exactly the volume 273 divisions at 0°. But —
M. Well?
THE EXPAhlSlON OF AIR BY HEAT. 213
P. I was going to ask something stupid. If I know
it for 273 divisions I can calculate it for any other number
by simple proportion.
M. Quite right. When 273 divisions measured at 0°
increase one division for each degree, one division will
increase by Y273) 3,nd for any number of degrees, which
may be called /, by divisions. You can draw this
^273
more easily if you use paper ruled in millimetres. There
is a net of lines on the paper which stand exactly i milli-
metre apart, and you do not need to measure, but you
can count the numbers directly.
P. But I must count the lines.
M, That is very easy; for every fifth and tenth line
is a little broader, and you need only write 10, 20, 30,
etc., on these lines.
P. Yes, that works very well.
M. Now tell me: what would be the effect of cooling
the gas below o°?
P. I think that its volume would decrease by V273 for
each degree.
M. Quite right. In our drawing you need only pro-
duce the line downwards towards the left in order to
represent volumes below 0°.
P. But I don't see what that means; the line gets
nearer and nearer the thermometer-line and finally
touches it. That must mean that the volume of the
gas becomes nothing, and if I produce it further, less
than nothing.
M. Quite right. At what temperature does that
happen ?
P. About -273°.
214 CONVERSATIONS ON CHEMISTRY.
M. Yes; if air loses Vgys of its volume for each degree,
at 273° below zero nothing is left.
P. Is that really the case?
M. I don't know, for the temperature —273° has
never been reached.
P. Why not?
M. It hasn't been found possible. The lowest tem-
perature ever obtained is about —260°, and from the
trouble which it has cost to lower the temperature, we
must conclude that it will be long before any attempt
to lower the temperature 10° more will be successful.
P. Has the air at — 260° really the small volume that
is shown on the diagram?
M. The volume is still smaller, but the reason is that
at — 190° air is no longer a gas, but condenses to a liquid.
P. Oh, then this part of the diagram has no meaning.
M. Yes it has. There are other gases, for example,
helium, which behave exactly as the diagram would in-
dicate, at the lowest temperatures. Helium has never
been liquefied; at lower temperatures it would be, no
doubt ; but we can imagine a gas that wouldn't and then it
would behave as is shown in the diagram.
P. Does the diagram apply to all gases ?
M. Yes, all gases behave exactly Hke air, and alter
their volume by Ysra of the volume which they occupy at
0°, for each change of one degree. Here we again have
a general law of nature, which makes it possible to pre-
dict the behaviour of very different substances. You
can say beforehand that if any substance is a gas, it will
expand by Yava of its volume for each degree.
P. That is very convenient.
M. You can see that all gases point to the temperature
— 273° as a limiting temperature. It is probable that it
THE EXPANSION OF AIR BY HEAT. 2l5
would be impossible to obtain lower temperatures than
— 273°. This would therefore be the lowest possible tem-
perature; you may remember we spoke of this before;
and if we mark the position of the mercury in the ther-
mometer at the melting-point of ice, with the number
273°,and the boiling-point of water with 373°, we should
probably never be obHged to calculate with negative
temperatures. We therefore call the temperature — 2 73°
the absolute zero, and the temperatures counted up-
wards from it are called absolute temperatures.
P What is the good of that?
M, It is very useful in many respects; chiefly in the
theory of heat, and the time hasn't come to explain that
to you. But I will tell you one thing. If the melting-
point of ice is made 273°, and the boiling-point of water
373°, these numbers have the same ratio to each other
as the volumes of air or of any other gas at these tem-
peratures.
P. Why is that?
M. You only need to look at your diagram.
P. Oh, I understand. The diagram has been made
from these numbers.
M. So the volumes of gases are proportional to their
absolute temperatures.
P. That's very neat. I didn't think I could have
learned so much from a simple drawing.
M. Do you see why? It is because in a diagram
everything is before your eyes, whereas in words or
calculations we can only discuss single points. You
must always try to represent general relations, or laws
of nature by means of diagrams.
P. I'd do it if I oilly knew how.
M, We shall have other examples.
« 1 6 CONyERSA TIONS ON CHEMISTR Y.
P. Please let me put one question before you stop.
You have been speaking again as if temperature was
the only reason why the volume of air alters, but I know
that pressure changes it too. What happens when both
change at the same time?
M. That is a very good question. You want to know
how to calculate the volume of a gas when both its tem-
perature and pressure change?
P. Yes.
M. Then calculate first the change that would take
place if only the pressure were altered without change of
temperature, and then the change which would be pro-
duced in the resulting volume by change of temperature
at constant pressure.
P. Why must I first calculate the change of pressure?
M. You might just as well calculate the change of
temperature first.
P. Should I get the same result?
M. Of course. The volume of the gas depends only
on the temperature and the pressure, and it is all one
in which order they are taken.
P. That looks right, but I don't feel quite sure about
it.
M. Let us take an example. Suppose we have 350 ccs.
of air at 18°, and that 74.8 cm. is the height of the barom-
eter, and that we wish to know what the volume will be
at 0° and 76.0 cms. That is the standard temperature
and pressure for measuring gases; they are called normal
temperature and pressure. Let us suppose first that
according to Boyle's law the volume varies inversely
as the pressure. Let us call the unknown volume at
76 cms. jy then ;y 1350 =74.8: 76.0.
P. Then ^'-344.
THE EXPANSION OF AIR BY HEAT. 217
M. Furthermore, the volume at 18° is to the volume
at 0° as 273+18 = 291 to 273. If you call the volume
at 0° X, you have the proportion —
P. :x; 1344 = 273 1291, therefore :r=323.
M. Quite right. Now you can reduce the volume
350 CCS. first to 0°, and then to 76 cms. pressure, and
convince yourself that you get the same answer.
P. We have been talking so long about air that I
have almost forgotten that these are supposed to be
lessons in chemistry.
M, What you have learnt holds for all gases. If you
have two equal volumes of any two gases at equal tem-
perature and pressure, their volumes always remain
equal, however you alter simultaneously the temperature
and pressure of the gases.
P. It is quite different from liquids, for water expands
differently from mercury.
M. Yes, gases show no peculiarities; they all behave
in the same way. You will see later that the behaviour
of gases is identical in many respects whatever their
chemical differences.
P. Are all gases colourless?
M. No, I have told you already that chlorine is greenish,
and iodine gas violet. But I must tell you that the
conformity is confined to the gaseous state. As soon
as a gas condenses to a liquid the differences appear
again, for one gas is more easily condensed than another.
The same is the case when gases dissolve in water or
other liquids. But as long as the substances are in
the state of gas, the uniformity of their external prop-
erties is maintained.
P. I'm glad you have answered my question. For
now I have learnt without knowing it that what you
2 1 8 CON VERS A TIONS ON CHE MIS TR Y.
have taught me about air appHes to all other gases. Do
vapours, Hke steam, obey the same laws?
M. Certainly there is no difference.
26. THE WATER IN THE AIR.
M. So far you have learnt about two of the constitu-
ents of air, oxygen and nitrogen, but these are not all;
there are others, among which water in the state of vapour
is important.
P. Yes, I wanted to ask you about that. The pressure
of the air is one atmosphere, and at that pressure
water boils at ioo°. How does it happen that water-
vapour can be present in air when it is much colder
than ioo°? Why doesn't all the water- vapour condense
to liquid water?
M. I am very glad you are taking such pains to learn,
for I have given you a hint how to answer that ques-
tion. It happens that when water evaporates only the
pressure of its own vapour counts, and not the pressure
of other gases and vapours which may be present at the
same time.
P. Please explain that to me.
M. Remember what I told you before about the
behaviour of water in a vacuum (page 176); it will
evaporate until the space is filled with vapour of a
definite density. Now if any other gas is present in
the same volume, for example, air or hydrogen, the
water-vapour will do exactly the same thing — it will
be formed until it has filled the space. Its pressure
will be added to the pressures which the other gases
exert, and the final pressure will be the sum of all.
THE IV^TER IN THE AIR. 219
Only the evaporation is somewhat slower, because the
vapour requires time to spread or diffuse throughout
the other gases.
P. I think I understand, but I should like to see it.
M. First of all, you can easily convince yourself that
ordinary air really contains water in the state of vapour.
You know that this water deposits upon cold objects in
the form of dew, and that jain falls because the water-
vapour of the air changes into liquid water when it is
cooled.
P. So water- vapour can be removed from air by cool-
ing it?
M. Quite easily. I close the neck of a small flask
with a cork, through which pass an entry and exit tube
(Fig. 43), and make a freezing- mixture with powdered ice
and salt, in the proportion of 3 to i, and surround the
flask with it. Then I only need to draw air from the
room through the flask for some time in order to collect
a considerable quantity of water in the form of ice in
the flask. If I let the flask warm up, of course I get
water.
P. But how can I make a stream of air pass through
the flask? It is tiresome to suck so long.
M. We will use our gas-holder for it (Fig. 28, page 149).
If we place the empty bottle on the floor, and connect
the other with it by means of an india-rubber tube,
when the water runs into the lower bottle it sucks air
into the upper one. We can regulate the rate with the
clip. And if we want to suck more air through, we
have only to reverse the bottles, and connect the upper
one with the under one by the tube.
P. That's capital. It didn't occur to me that you
could suck with it as well as blow.
220
CONVERSATIONS ON CHEMISTRY.
M. Now the experiment has been going long enough;
you see a quantity of frost has condensed in the flask.
P. Is cooling the air the only way to take water out
of it?
Fig. 43-
M. No, it can be done in other ways. There are
many substances which combine so easily with water
that we only need to lead moist air over them to remove
the moisture. Caustic soda, which you have already
seen, is such a substance (page 70); another is a salt
named chloride of calcium, which is made in large quan-
tities as a by-product in chemical works. When it is
dried or fused it takes water so quickly from the air
that a piece left in an open dish changes in half an hour
into a liquid drop. Air and other gases can be very
easily dried by its help.
P. How?
THE lVy4TER IN THE AIR.
221
M. The salt is placed in a specially shaped tube (Fig.
44) and the gases that we wish to dry are led through it.
If you cannot blow such tubes for yourself, you need
only close a wide tube at both ends, with perforated
corks having narrow tubes through them; only don't
Fig. 44.
forget to close the end of the tube with cotton-wool in
order that particles of dust from the salt may not be
carried along by the stream of air. If you weigh such a
tube exactly, and then pass a measured quantity of
air through it and weigh it again, you can find out how
much water was in the air.
P. I shall try that.
M. You will not find much unless you pass a couple of
dozen litres through it.
P. How much water is there in the air ?
M. It varies greatly. It depends on the temperature
as well as on the source of the air. Do you remember
what I have just told you about the evaporation of water
in the air (see page 218).
P. Yes, exactly as much evaporates as if there were
no air present in the space.
M, Quite right. You know then that the pressure
of the vapour, and consequently the amount in a given
volume, will be greater the higher the temperature. Here
2 22 CONFERS A TIONS ON CHE MIS TR Y,
is a table which shows you how many grams of water-
vapour are present in a Htre of air when it stands in
contact with liquid water, or, as we say, when it is satu-
rated with water-vapour. A litre of saturated air con-
tains
At o° o .0049-gram water-vapour
' 5° 0.0068 "
* 10° 0.0094 " *'
* 15° 0.0127 " "
' 20° 0.0171 "
* 25° 0.0228 " "
P. The word saturated is the same that you use for
solutions.
M. It is really the same thing, for it means that the
air cannot take up any more water-vapour.
P. But it can take up less?
M. Certainly, that is also the case with solutions.
Ordinary air like what is in the room is almost always
unsaturated; it is only saturated when rain or mist is
present. The proportion between the amount of water
in the air and the quantity required for saturation is
called the hygrometric state of the air. When air, for
example, at 20° contains 0.0140 gram of water per Htre
• . . t 0.0140
its moisture is equal to = 0.82, or 82 per cent,
^ 0.0171 ' ^ '
because, according to the table, it could contain 0.0171
gram. Air generally contains about 70 per cent of
moisture; if it contains 50 per cent we call it dry, and
we call 90 per cent damp.
P. I understand that.
M. Now look at the table again; if the temperature
rises 10° the amount of water is nearly doubled. A
sample of air which is only half saturated at 20° is almost
THE IVATER IN THE AIR. 223
completely saturated at 10°, and ordinary air with 70
per cent of moisture, if it is cooled 10°, will part with a
good lot of its water in the Hquid state. That is the
reason of rain.
P. Numbers make such a lot of things clearer than
words. But why should rain come and not mist?
M. That depends upon how much water has to be
separated. If there is only a little, the very small drop-
lets which are formed do not unite to large drops and the
result is fog; in other cases rain occurs, but fog and
mist always precede rain; we call the fog which occurs
in the upper air a cloud.
P. How do you know that clouds are only mist?
M. The tops of mountains are often hidden by clouds,
and when you cKmb them you find the clouds are nothing
but mist.
P. Please tell me how it happens that air is not com-
pletely saturated with water-vapour. It is always touch-
ing water, either the sea or ponds on the land.
M. That depends on its motion. Suppose you had
the air saturated in one place, if it moves to a warmer
place it will become unsaturated as you can see from
the table, or if it moves to a colder place it loses a portion
of its water in the form of rain. And when it recovers
its original temperature, it is unsaturated again. So
whatever happens, when it changes it can only change
in one direction, in becoming less saturated.
P. That is far simpler than I thought it was.
2 24 CONVERSATIONS ON CHEMISTRY.
27. CARBON.
M. The element carbon is as widely distributed and
as important as oxygen, hydrogen, and nitrogen. You
already know that ordinary charcoal is one form of that
element.
P. I thought you would be speaking of carbon to-day,
and so I looked at a piece of charcoal. I noticed one
thing; you can see the rings in the charcoal just as in
the wood.
M. Yes, you can see the rings which show the number
of years of growth, and besides that you can see under a
microscope the single cells of which the wood consisted.
P. But surely wood doesn't consist only of carbon?
M. No; it is a compound of carbon, hydrogen, and
oxygen. In charcoal-burning as it is called the wood is
slowly heated, and carbon alone remains, for the other
two elements are expelled. But as carbon melts only
at a very high temperature, which is not nearly reached
in charcoal-burning, the remaining charcoal retains the
form of the cells of which the wood consisted. More-
over, wood charcoal is not pure carbon. You see that
when it burns, for ashes always remain, while pure
carbon leaves no residue on burning.
P. Is there such a thing as pure carbon?
M. Certainly ; ignited lampblack is nearly pure carbon.
You know that lampblack is a very fine black powder.
P. You said before that almost all pure substances
form crystals, but lampblack doesn't look crystaUine.
M. Nor is it. Such substances are called amorphous,
which means without shape. Lampblack is amorphous
carbon; so is wood charcoal, only it is impure.
CARBON. 225
P. Is coal carbon, too ?
M. No, ordinary coal and its varieties, anthracite,
brown coal, and peat, are. all chemical compounds which
contain a large percentage of carbon; anthracite con-
tains most; peat, least. They all owe their origin to
plants. Indeed, in coal the remains of plants are not
infrequent; in brown coal they can be seen even more
distinctly, and peat sometimes consists almost entirely
of stems and roots. These materials, owing to their being
buried a long time in the earth, have undergone almost
the same changes, as wood undergoes on being carbonized
by heat, only the change is a much slower one.
P. Now I begin to see why you told me that carbon
was such an important element. All fuel consists of car-
bon.
M. Quite right. But fuel is used not merely for heat-
ing, but for all sorts of other purposes. All machines
except those driven by running water, like water-mills,
are driven by means of carbon; moreover, in chemical
works and in works in which iron and other metals
are smelted, the processes are all carried out by help
of carbon; indeed, the progress of our civilization may
be said to depend on carbon.
P. Why is that? I mean why is carbon required for
all such purposes?
if . By the burning of carbon a large amount of work
is made available, which generally appears in the form of
heat. By help of such work machines are set in motion.
Chemical processes are carried on which would not go
of their own accord; in short, carbon places at our dis-
posal quantities of energy which we use for all sorts of
work.
P. Why, you said the same about oxygen.
226 CONVERSATIONS ON CHEMISTRY.
M. Energy is only liberated when carbon and oxygen
combine with each other, that is, when carbon burns.
You see the carbon is as necessary as the oxygen.
P. And because oxygen is a gas, it is everywhere
round us, but carbon must be bought because it is a
solid.
M. Well done! that is a good remark. What you
say is quite right. But you see this gives people the
power of putting energy where it is wanted. If carbon
were all round us in the form of gas, as air is, you might
perhaps be able to set the air on fire, but you couldn't
have a fire in a fire-place.
P. They would explode.
M. Quite right. But don't let us speculate; let us
think of what actually exists. Carbon is the most im-
portant source of energy at the disposal of our industries.
Notice this; when carbon is burned, the products of its
combustion are sent up the chimney as fast as possible,
only people try to keep in the heat which is produced
at the same time as thoroughly as possible. It is evi-
dent then that coal is bought not on account of the carbon
it contains, but on account of the energy which it can
give out.
P. It never struck me in that way before. But I see
that it must be right.
M. You know that a steamer or a locomotive must
carry coal with it. Each can go only as far as the coal
will permit. If the coal gives out the engine stops.
And so there are islands on the ocean, and stations on
the coast, where ships can buy more energy in the form
of coal.
P. But if I were to row a boat I shouldn't need to burn
any coal.
CARBON. 227
M. You know the answer quite well yourself. Think
of what I told you about the use of oxygen in supporting
life.
P. Yes, I remember that food does the same as coal.
But does food consist of carbon?
M. All food contains carbon, and when it burns it gives
up energy, just as when it passes through our bodies.
Food consists of compounds of carbon, hydrogen, and
oxygen; sometimes it contains nitrogen too.
F. Yes, I remember. Foods that give a disagreeable
smell on burning contain nitiogen.
M. Yes. As food is also used for the building up of
the body, all substances of which the bodies of animals
and plants consist contain carbon. Such substances
are called organic compounds^ because animals and plants
are called organisms.
P. Are there many organic compounds?
M. We know more than a hundred thousand, and new
ones are being discovered every day.
P. How can any one remember them?
M. No one can, of course; but that doesn't matter;
there are dictionaries in which accurate descriptions of all
these compounds are to be found.
P. Do other elements form as many compounds?
M. Not by a long way. And so the chemistry of car-
bon compounds is treated separately from that of the
other elements, and called organic chemistry, while the
chemistry of other substances is called inorganic chemistry.
P. Isn't that rather an arbitrary division?
M. Not so much as it looks. Carbon compounds
have certain general properties which bring them natu-
rally into one group. Moreover, certain simple carbon
compounds are treated of under the head of inorganic
2 28 CONVERSATIONS ON CHEMISTRY.
chemistry, because carbon occurs in many minerals and
rocks.
P. Yes, as coal and peat.
M. No, in other chemical compounds. Marble and
chalk contain carbon. But we shall come to that later;
in the mean time let us consider the uncombined element.
We must now consider a new phenomenon which you
ought to learn about. Do you know that the diamond
is nothing but carbon ?
P. Yes, because it can be burned when it is heated
very hot.
M. That is not a sufficient reason, because many other
substances can be burned at a high temperature although
they are not carbon. Oxygen, you know, unites with
most other elements.
P. Yes, but when a diamond burns I have read that
nothing is left.
M. That is a better test. We could infer that the
oxygen compound or the oxide of the element or elements
of which the diamond consists is volatile. But carbon
is not the only element which leaves no residue in burning.
Sulphur and hydrogen give volatile oxides.
P. It must depend then upon what is formed.
M. Quite right, now you are getting nearer it. When
carbon burns a gas is evolved which is called carbon
dioxide; we have talked about it already (page 68).
It can be easily recognized because it gives a white pre-
cipitate with lime-water, and makes clear lime-water
milky. To remind you, I will make the experiment
somewhat differently; here is a fragment of charcoal in
a glass tube. Now I will heat the place wh^e it lies
and pass air over it from the gas-holder (page 220).
The tube is bent so that the end dips under lime-water
CARBON.
229
in the glass. Now the carbon begins to glow, and you
see the lime-water is turning turbid.
P. If I were to heat a diamond instead of charcoal,
would it burn and turn the lime-water milky?
M. Yes, it would, only you couldn't make the experi-
ment in an ordinary glass tube, because the diamond
would not catch fire at so low a temperature; the tube
would melt. Moreover, you would require to use pure
oxygen, for the diamond would then burn more easily.
Fig. 45.
P. Yes, I see the proof that the diamond is really
carbon.
M. Stop a little; not quite so quick. You would only
prove that the diamond contains carbon, but not that
it contains nothing but carbon. How could you find out
that there are no other elements present?
P. I don't quite understand you.
M. Look here. I shall repeat my former experiment
with a piece of wood. It catches fire, too, and the lime-
water again turns milky. Yet I cannot say that the
wood is carbon, but only that it contains carbon; for
it also contains hydrogen and oxygen.
P. Let me think. Now I have it; water must be pro-
duced by the burning of the hydrogen. If carbon di-
230 CONVERSATIONS ON CHEMISTRY,
oxide is the only product, we know that the substance
contains only carbon.
M. That is more nearly correct, but not quite. The
diamond might be a compound of carbon with less
oxygen than is contained in carbon dioxide. Such a
compoujid on burning would give only carbon dioxide,
and yet it would not consist of carbon alone.
P. Is there such a compound?
M. Certainly, but it is not a soHd hard substance like
a diamond, but a gas.
P. Then there's no risk of mistaking it for a diamond ?
M. You are trying to evade the point. That is unad-
visable, because you will miss an opportunity of learning
something.
P. I must say I don't understand the question of the
diamond.
M. When carbon bums, 3 parts by weight of carbon
always give 11 parts of carbon dioxide, for it com-
bines with 8 parts of oxygen; now with the diamond
that is the proportion. If the diamond contained some
element besides carbon it would give less carbon dioxide,
and indeed the amount would be proportional to the
amount of carbon it contains.
P. Then wood must give much less carbon dioxide than
charcoal.
M. So it does; 3 parts by weight of wood give at most
4j parts of carbon dioxide.
P. And no single substance is known which gives
more?
M. Not one. But there is another substance which
gives exactly the same amount, namely, graphite, of
which ordinary lead pencils are made.
P. Then it must be carbon?
CARBON. 231
M. Yes, we must therefore say that carbon appears
in three different forms: charcoal, diamond, graphite.
P. But I don't understand that. How can one and
the same substance exist in three different forms, and
why do people not make diamonds from charcoal if it
contains only carbon?
M. That is a very good question and I will answer
you as well as I can. You already know that one and
the same substance, for example, water, can exist in
different forms. Water, indeed, exists in three, namely,
ice, water, and steam.
P. Yes, there are three forms, but carbon, diamond,
and graphite are all solid bodies. But if all three could
be changed into each other by heating or cooling then I
would beheve it. But they can all exist together at the
same temperature.
M. You are quite right. Nevertheless charcoal can
really be changed into graphite; it takes place at a very
high temperature.
P. Can you show me that?
M. It is not so difficult. The carbon rods used for
electric lamps consist of ordinary carbon. The next
time the workman puts in new carbons ask him for one
of the old ends. You will see that the points have
become grey and smooth and shine something Kke a
metal; they have changed into graphite. The carbon
threads of an electric glow lamp have undergone the
same change owing to the high temperature at which
they have been kept. Indeed, they are made of cotton
carbonized, and after they have served their purpose they
become grey and shining like graphite.
P. Next time a lamp burns out I will ask for it and
break it open.
232 CONVERSATIONS ON CHEMISTRY.
M. Take care that you don't lose the very fine thread.
P. Can diamond also be changed into graphite?
M. Yes, in exactly the same way, by heating it strongly.
P. And can the change be reversed?
M. Graphite can only be changed into ordinary carbon
by a roundabout process. It has to be made into com-
pounds, and the carbon separated again from them.
P. I don't quite understand that.
M. I will not try to explain it to you now, since sub-
stances must be used which you do not know about.
You must be contented in the mean time in believing
that it is possible.
P. But how is it with diamonds? Can they be made
from charcoal or graphite?
M. That is also possible.
P. Then why aren't diamonds cheap?
M. There is no danger of that, because only minute
diamonds can be made, and in very small quantity.
P. But why is that? Charcoal is cheap enough.
M. Yes, that brings us again to our general question.
I compared the three states of carbon with the forms
of matter. Now carbon is also able to become liquid
and gaseous, so that the ordinary forms of matter are
known in its case.
P. Liquid and gaseous carbon!
M. Yes, but a very high temperature is necessary —
more than 3000°; however, it can be reached by the
electric current. You see then that carbon can exist
in the gaseous, Hquid, and soUd states. Carbon is known
not only in three, but in five, different states.
P. So that's how it is. Yes, I see. Just as water
can be changed to steam by heating it, so charcoal changes
to graphite by heating; no, I don't see yet. When
C/iRBON. 233
graphite cools, it doesn't go back to charcoal, but remains
as it is.
M. Yes, that is the most difficult side of the question,
but I think I can explain it to you. You know that
water changes into ice at 0°. Do you remember what I
told you about super-cooHng (page 164).
P. Yes, that water can be cooled below 0°, and remains
liquid if no ice is present.
M. Right. I have here a sealed glass tube containing
water into which no ice can enter. I now place the tube
in a mixture of ice and water which of course is at 0°,
and I can leave it as long as I like; ice will never be
formed.
P. That won't do, you must cool it below 0°.
M. Quite right. Now if I add some salt the tem-
perature will fall below 0°. Half a teaspoonful will do;
the thermometer shows —4°, and there is no sign of the
water freezing.
P. But if you were to leave it for a long time ?
M. Nothing would happen. If I were to add more
salt so as to lower the temperature to —10°, and were
then to shake it violently ice would be formed.
P. Yes, I see that.
M. Now with a diamond, you must think in the
same way. The conditions of our experiments are
such that the diamond cannot be formed. In order
to produce it a very great pressure and a very high
temperature are necessary. These conditions are diffi-
cult to obtain, and hence it is very difficult to make
diamonds.
P. Yes, I can understand that. But why is carbon
the only substance which behaves like that?
M. Carbon is not the only substance. You will soon
234 CONVERSATIONS ON CHEMISTRY.
become acquainted with other substances which also exist
in several solid forms.
P. Are such different forms only known in the case
of solid substances?
M, For the most part. Such substances are called
allotropic. Charcoal, diamond, and graphite are allo-
tropic forms of carbon.
P. Now I think I understand it a little. But I want
to ask one question: What do these differences depend
on — what are they connected with?
M. With the differences in the amount of work or
energy in the substances. Just as work is required to
change ice into water, or water into steam, so energy
is required to change charcoal into diamond; and no
second substance takes part in this change in either
case.
P, Could we not regard energy as a kind of chemical
element which combines with a substance, and gives it
different properties?
M. That is one way of looking at it, but energy pos-
sesses no weight, and therefore during such allotropic
changes there is no change of weight.
P. Now I think I understand everything.
28. CARBON MONOXIDE.
M. You have seen, several times, what happens when
carbon is burned.
P. Yes, a gas is formed named carbon dioxide, which
consists of carbon and oxygen. Why is it called dioxide ?
M. Because there is another compound of the two
elements which is called carbon monoxide. The dioxide
CARBON MONOXIDE, 235
contains twice as much oxygen as the monoxide. The
syllables mon and di are Greek prefixes meaning one
and two.
P. What is carbon monoxide like?
M. It is a colourless gas, but differs from the dioxide
by being combustible. Moreover, it is very poisonous.
P. Can I see it?
M. Yes, if you can speak of seeing a gas; it is colour-
less so that you cannot distinguish it from air in appear-
ance ; its density and its other physical properties resemble
those of nitrogen. But you have often seen it burning.
P. When and where?
M. You have often seen coal burning in the fire. At
first you know it gives out a bright flame which comes
from the burning of compounds of carbon and hydrogen
which resemble coal-gas, and which give the flame its
brightness.
P. Yes, of course.
M. After all the coal glows red-hot the flame changes
its appearance and becomes pale blue in colour.
P. Yes, I have noticed that. It looks like the flame
of a spirit-lamp.
M. Yes, that is the flame of carbon monoxide burning.
At first the oxygen of the air combines with the carbon,
of the coal, to form dioxide; but the dioxide in passing
through the red-hot coal combines with the carbon and
forms carbon monoxide; then the carbon monoxide
burns at the back of the fire to carbon dioxide when it
comes in contact with more oxygen.
P. I must look more carefully at that.
M. Do so, and think of this. Carbon monoxide is
like nitrogen because it has no smell; but, as I told you,
it is very poisonous. If it escapes into the room much
^$6 CONVERSATIONS ON CHEMISTRY.
harm may be done, and every year people die of poisoning
by carbon monoxide.
P. How does that happen?
M. It seldom happens with an open fire unless the
damper in the chimney is shut. But in a stove, if a
sufficient quantity of air is not let in to burn the carbon
to dioxide, the monoxide is formed, which may escape
into the room and poison the people in it.
P. But surely the amount of carbon monoxide in a
room must be very small, because the volume of the
room is so very much larger than that of the stove, and
besides air is always entering through the cracks of the
door- and the window-frames.
M. Quite right, but, unfortunately, carbon monoxide
is absorbed by the blood even when very little is con-
tained in the air. People who are poisoned by carbon
monoxide show no signs of suffocation, but only become
dull and sleepy and get headaches, and do not realize
what it is and try to escape.
P. Can anything be done with people who are poisoned ?
M. The best way is to take them as quickly as possible
into the open air, and make them draw deep breaths.
If they are far gone oxygen may be given if it is at hand,
or artificial respiration may be applied in the same way
as for the recovery of the drowning, by moving the arms
up and down regularly. Don't forget that coal-gas
generally contains a good deal of carbon monoxide, which
makes it poisonous. But in general the smell is suffi-
cient warning, although it is due to other constituents
of the coal-gas.
P. Isn't it curious that a compound of carbon and
oxygen should be poisonous when neither of the elements is
poisonous, and when our bodies largely consist of them ?
CARBON DIOXIDE. 237
M. No, it is only another example of the fact that the
properties of compounds are entirely different from
those of their elements. I remember telling you before
that it isn't correct to speak as if the elements were
contained in their compounds.
P. Yes, I remember, too, but it is very difficult to
change one's ordinary way of speaking.
29. CARBON DIOXIDE.
M. Do you remember what we have learned about
carbon dioxide?
P. Yes, it is formed when charcoal bums, or when
any substances containing carbon are burnt. It can be
tested for with lime-water.
M. You have remembered that very well. What does
the lime-water look like after it has been treated with
carbon dioxide?
P. It becomes milky.
M, Yes. In the chemical language we say that a
white precipitate is formed.
P. What is precipitated?
M. If you let it stand the milkiness would settle to
the, bottom as a white layer, for it is heavier than water.
A solid substance which is produced in a liquid by a
chemical process is called a precipitate. What does
carbon dioxide look like?
P. A colourless gas.
M. Yes. It has the pecuhar property of being heavier
than air and so it behaves in a manner different from
hydrogen, for it sinks in air instead of rising like hydrogen.
P. I should like to see that.
238 CON VERSA TIONS ON CHE MIS TR Y,
M. We must first make some carbon dioxide for that
purpose. I shall use a flask exactly Hke the one I used
for making hydrogen (Fig. 24, page 135) only instead
of putting zinc in the flask I use chalk or marble; the
funnel contains dilute hydrochloric acid. You see that
it begins to froth as soon as I let hydrochloric acid into
the flask; the gas which is evolved is carbon dioxide.
P. What does the hydrochloric acid do to the chalk ?
M. I won't explain that until you know more; but you
will soon learn. We shall first make sure that the gas
which is evolved is really carbon dioxide. I am passing
it into an empty flask, and now I pour in some lime-water
and shake it.
P. Yes, I see, that is the white precipitate.
M. This experiment shows you at once that carbon
dioxide is heavier than air, for it has stayed in the flask.
But I can show you this better by filling two test-tubes
with the gas just as we did with hydrogen (page 137)
and leaving one witH its mouth upwards and the other
with its mouth downwards. This time it is the one with
its mouth upwards that remains full. How could you
find that out?
P. I could test with lime-water.
M. You could do it even more simply. Carbon di-
oxide puts out a burning splinter. Look, I thrust a burn-
ing match up into the tube with its mouth downwards ; it
goes on burning. But it goes out when I put it into
the tube with its mouth upwards.
P. Then the same test does for carbon dioxide as for
nitrogen.
M. Yes, so far as the burning splinter is concerned.
But they behave differently with lime-water, for nitrogen
gives no precipitate with it. It is not uncommon for two
CARBON DIOXIDE. 239
substances to behave similarly towards one test, but if
they differ in any respect they must be different sub-
stances. There are many other differences between
these gases. Carbon dioxide, for instance, is heavier
than nitrogen.
P. Why did the splinter go out in carbon dioxide?
Doesn't the dioxide contain oxygen?
M. That is a good question. You know the splinter
consists largely of carbon ; now that carbon would require
to displace the carbon in the carbon dioxide, in which
it is already combined with oxygen. It is almost as if
you were trying to raise yourself into the air.
P. Oh!
M. But other substances can take away oxygen from
carbon dioxide. You have seen magnesium ribbon which
burns so brightly. I fill a flask with carbon dioxide —
P. Why don't you collect the gas over water?
M. That is not necessary ; it is so heavy that it stays at
the bottom of the flask. And I know that the flask is
full because it puts out a burning spail when I hold it
to the mouth; the flask is full, and the carbon dioxide
is running over.
P. That's an easy way of doing it! Let me try; yes,
now the flask is full.
M. Now I fold several pieces of magnesium ribbon
together (for a single piece goes out too easily), Hght it,
and dip it into the carbon dioxide.
P. It hisses and sparkles!
M. You see that it burns quite differently from what
it did in the air. There are white and black particles;
the white particles are oxide of magnesium, the black
particles are the carbon from the carbon dioxide.
P, Oh, mav I look at that?
24© CONVERSATIONS ON CHEMISTRY.
M. Wait a little. I have poured some hydrochloric
acid on it; it dissolves the magnesium oxide, and leaves
the carbon.
P. Yes, it has become quite black. What made that
frothing?
M. It was a little piece of metallic magnesium, which
acts like zinc upon the hydrochloric acid, and evolves
hydrogen. Now I will show you another property of
carbon dioxide. I fill a flask with the gas over water, let
a httle more water enter, close the mouth with my thumb,
and shake. You see my thumb sticks to the mouth, as
if it were sucked in; that shows that the pressure in the
flask has decreased. When I dip the neck under water
and remove my thumb, a good deal of water enters. Now
I can repeat this till at last the flask is almost complete-
ly filled with water. What does this experiment show
us?
P. That carbon dioxide is dissolved in the water.
M. Yes, it is pretty soluble. A litre of water at the
ordinary temperature absorbs nearly a litre of carbon
dioxide; it absorbs a little more when it is cold, and
less when it is warm.
P. Is that not the way soda-water is made? I think
I remember your telling me that.
M. Yes, soda-water is a solution of carbon dioxide
in water.
P. But doesn't it contain soda?
M. It used to contain soda, but now it is merely a
solution of carbonic acid in water. The name carbonic
acid, although it is commonly used, should not be applied
to the gas, but only to its solution in water. Why does
soda-water effervesce? Do you remember what I told
you about that?
P. Yes, you told me that the bottles are filled at a high
CARBON DIOXIDE. 241
pressure with the gas, and when they are open the pres-
sure decreases and the gas comes out. I remember,
too, that you said that the same volume of gas is dissolved
whatever the pressure is.
M. Quite right. You learned that at any given tem-
perature the weights of a gas which fill a given volume
are proportional —
P. To the pressures?
M. Yes. If equal volumes are always dissolved at dif-
ferent pressures, what will the weights be proportional to ?
P. To the pressures.
M. Quite right. So at different pressures different
weights of gas will be dissolved, and these weights are
proportional to the pressures. Soda-water has generally
a pressure of four atmospheres; therefore it contains
four times as much carbon dioxide as it can retain under
a pressure of one atmosphere. This excess escapes on
opening the bottle, and produces the frothing.
P. Some other liquids foam; for instance, beer. Does
that depend upon carbon dioxide, too?
M. Yes, but the gas is not pumped into the beer,
but is formed in the beer from malt, and remains dis-
solved in the liquid.
P. Then where does it come from?
M. There is sugar in malt, and by the action of yeast
this is decomposed into alcohol, which gives the beer its
intoxicating properties, and into carbon dioxide, some
of which is evolved. In beer- cellars they sometimes use
iron bottles filled with liquid carbon dioxide for driving
the beer out of the casks.
P. Liquid carbon dioxide?
M. Yes, when carbon dioxide is compressed with a
powerful pump it turns Hquid like water, and indeed has
almost the same appearance.
242 CONVERSATIONS ON CHEMISTRY.
P. It must be a very strong pump.
M. The pressure depends on the temperature. At o°,
35.4 atmospheres are required; at 20°, 58.8, but at —80°
carbon dioxide Hquefies at i atmosphere pressure. Liquid
carbon dioxide boils at —80°. It behaves exactly like
water, for its vapour has a higher pressure the higher the
temperature. Only the corresponding temperatures for
carbon dioxide He much lower.
P. Should we call carbon dioxide a vapour?
M.' You may if you like.
P. Couldn't you bring me some liquid carbon dioxide
home in a bottle to see what it is like?
M. That would be impossible, for when it is allowed
to escape, out of the steel bottle, it becomes solid like
snow.
P. How is that?
M. You know that on boiling a liquid, heat is absorbed,
all liquids behave in the same way in this respect, and
carbon dioxide is no exception. As soon as liquid carbon
dioxide is exposed to air which has only one atmos-
phere pressure, it begins to boil violently, and so much
heat is absorbed by the portion which evaporates that
the residue freezes.
P. Then it should be possible to freeze water by
boiling it! Surely that could never be done!
M. It is not difficult, only care must be taken that the
water shall boil below 0°; and to accomplish that the
pressure must be very low. As a matter of fact water
can be frozen if it is brought into a space as free from
air as possible; and then it behaves exactly as I have
told you that carbon dioxide does. Indeed, there are
ice machines in which ice can be made in summer by
this process. You see carbon dioxide resembles water
CARBON DIOXIDE. 243
in existing in all three forms. Liquid carbon dioxide
has become a valuable article of commerce for aerating
water and for forcing beer out of casks, and if you look
you will often see the steel flasks filled with liquid carbon
dioxide being carted about on the streets.
P. Where does it chiefly come from?
M. It pours out of the earth. In many places, espe-
cially where there are or have been volcanoes, pure
carbon dioxide issues continuously from the soil. When
it comes in contact with subterraneous springs, the water
becomes saturated with the gas and escapes as carbonated
or sparkling water.
P. Why does it taste sour?
M. A solution of carbon dioxide has an acid taste.
P. Is that why it is called carbonic acid?
M. That has to do with it. Sometimes carbon dioxide
issues from the earth as a gas, and can be compressed
into steel flasks with the help of powerful pumps. There
are such carbon dioxide wells at Naples, in the neighbour-
hood of Vesuvius. There is a cave, the floor of which
is somewhat depressed, into which the gas pours until
it fills the depression nearly two feet, and the gas flows
out over the floor just as if it were water. People can
walk about in this grotto without danger, because their
heads are above the level of the carbon dioxide, but
dogs are suffocated, as they are at a lower level. That
is the well-known ''Grotto del Cane," or *'Cave of the
Dog."
P. Do they really let dogs suffocate in it?
M. No, they bring them out before they are dead,
and revive them by splashing them with water.
P. How cruel! Why are dogs suffocated by carbon
dioxide ?
244 CONVERSATIONS ON CHEMISTRY.
M. For the same reason that they die in nitrogen;
because they can get no oxygen to breathe. Carbon
dioxide isn't really a poison any more than nitrogen,
because it is always present in our lungs.
P. How does it get there?
M. Out of the blood. I have already told you that
the food which we eat contains carbon and that it is
burnt in our tissues by means of the oxygen which the
blood leads to it. It burns to carbon dioxide just as in
ordinar}^ combustion; the gas is absorbed by the blood,
and we breathe it out from our lungs along with nitrogen.
P. So carbon dioxide is present in the air which I
breathe out?
M. Certainly; blow some air through a glass tube
into lime-water.
P. So it is, the lime-water becomes milky, and there is
a white precipitate. How much I have to think about !
30. THE SUN.
P. I have been puzzling my head ever since the last
lesson. I know now that carbon dioxide is produced
by combustion, by breathing, and by decay, and that
in some places it streams out of the earth. It must all
collect in the air, and accumulate. Isn't the air full of
carbon dioxide?
M, There is always some in the air, but not very
much; only about 4 parts in 10,000. More is present in
close rooms when much carbon dioxide has been produced
by breathing or by the burning of gas. You can easily
recognize it by exposing some lime-water to the air in the
room, for it will become covered over with a white scum.
THE SUN.
^45
P. Covered over? Oh, I see; because the carbon
dioxide can only act on the surface of the water. But
what becomes of all the carbon dioxide that is poured
into the air? Perhaps the volume of the air is so great
that the carbon dioxide makes no difference.
M. That is not the reason. As a matter of fact,
there is a state of equilibrium in which the air loses as
much carbon dioxide as it receives.
P. What becomes of it then?
M. Plants absorb it. They decompose it in such a
manner that the carbon remains in the plant, and helps
to form its tissues, while the oxygen is returned to the
air as a gas.
P. Can plants really make oxygen? How can I see
that?
M. That is not difficult. We take a large glass funnel,
fill it with fresh green leaves, and place it upside down
in a wide vessel full of
fresh water. Then we
sink it so deep as to fill it
completely with water, and
we close the opening with
a cork. Then we expose it
to sunlight (Fig. 46).
P. Let me help you to
lift the pail.
M. You needn't trouble;
I push a plate below the funnel and lift out both together;
the water will not run out of the funnel. When the sun
shines on it, you sec gas bubbles rising, which collect at
the top.
P. Isn't that only the gas which has been dissolved in
the water, and which escapes when it is heated (page 122) ?
Fig. 46.
246 CONyERSATIOm ON CHEMISTRY.
M. No, the water doesn't grow warm so quickly. W^
shall leave it standing in the sun till we have collected
some cubic centimetres of gas. Then we will put the
funnel back in the pail, and hold it so that the water
stands at the same level inside and outside; now we can
take out the cork, and test for oxygen by means of a
glowing spail.
P. That is a beautiful experiment. I shall think of
plants quite differently now. What a lot of good they
do! I should never have thought it, for, by breathing
and burning, all the oxygen in the air would be used up
at last. Plants give us it back again.
M. You see that we owe a debt to plants because they
not only serve as food, but also because they restore us
the oxygen with which we bum our food.
P. I don't quite understand that. I eat as much
meat as vegetables.
M. But the animals whose flesh we eat live upon
plants. We never eat carnivorous animals. But if we
did, these eat graminivorous animals, so that in the long
run man and animals are nourished by plants.
P. Yes, I see that. But if plants restore the oxygen
to the air, air in the fields and woods must contain much
more oxygen. Perhaps that is the reason that the air
feels so fresh, and that it is healthy to live in the country.
M. No, that is not the reason. The difference between
the amount of oxygen in country air and in town air is
very small — it can hardly be detected.
P. How is that? Does it not contradict what you
have just told me?
M, The air is in perpetual motion, and it is mixed up
so rapidly that the differences quickly disappear. Even
a very moderate wind travels a mile in five minutes.
THE SUN. 247
You can think how quickly the air reaches the town
from the wood, and vice versa.
P. But above the sea?
M. There is no difference. Not only animals, but also
myriads of small plants hve in the sea. They all act in
the same manner, only they do not decompose the carbon
dioxide in the air, but that which is dissolved in the
water, and they restore the oxygen in solution. The
fishes and other sea animals make use of it, for they too
must derive their energy from the combustion of their
food.
P. Yes, they breathe through gills. What are gills ?
M. They drive the oxygenated water through struc-
tures which are permeated with blood-vessels just like
the lungs, and in which the carbon dioxide of their tissues
is exchanged for oxygen.
P. Just in the same way as with animals that breath'.*
air, except that water takes the place of air.
M. Quite right ; and there are still simpler lower animals
in which the water penetrates straight into their tissues.
P. It all goes round in a circle; what the animals do
not want the plants take up, and what they throw out
the animals use. Does the same happen with nitrogen ?
M. Yes; only nitrogen, as I have told you, must
always remain combined (page 187).
P. I remember; and if the nitrogen becomes free, it is
again made to combine in the soil. How wonderful!
But tell me one thing; I want to ask you why the leaves
must stand in the sun before they give up oxygen ?
M. You should be able to answer that yourself. When
carbon burns to carbon dioxide, much heat is liberated.
P. Of course, and that is the source of the work done
by machines and animals.
248 CONVERSATIONS ON CHEMISTRY.
M. Then in order to decompose the carbon dioxide
again, the same work must be done on it which was
Hberated vv^hen the carbon and oxygen combine. Where
do the plants get this work?
P. I haven't thought of that. You said something
about the sun; do they get it from the sun?
M. Of course they do. Plants lead a double life.
On the one hand they must work exactly Uke animals.
They must pump water, they must grow in size, they
must form buds and frui'.. They can't make this work
out of nothing; they must take it from somewhere by
consuming food. Now they differ from animals in this:
they make their own food, and they derive the necessary
work or energy from sunlight.
P. You say that plants derive their energy from
food Uke animals. Then they must give out carbon
dioxide ?
M. So they do. And that is what their double life
consists in. For the work that they carry out as animals
they derive the necessary energy from combustion. But
they collect this energv from sunlight; indeed they must
collect far more than they give out so as to have a reserve
for the dark. And so they always evolve carbon dioxide ;
but that can only be detected in the dark, for in sunlight
oxygen is evolved at the same time, and its amount is
far more than that of the carbon dioxide.
P. How do plants collect the energy from the sun ?
M. We do not know much about that. So far as we
know only green plants can do so ; colourless plants like
fungi and moulds Hve like animals on plant-food; for
example, rich soil, decomposing vegetable matter, and so
on. We do not know what becomes of the carbon
dioxide in the leaves where the energy is stored up; we
THE SUN 249
only know that the first product which we can detect is
starch. You must look on the green cells of plants as
little chemical laboratories, in which are prepared the
substances which the plant requires, and which are
fitted with arrangements to change sunlight or the
radiant energy of the sun into the energy of chemical
compounds.
P. Then does our life really depend upon the sun?
I remember that you told me (page 180) that the motion
of the water and the air on the surface of the earth was
caused by the heat of the sun. Really everything which
takes place upon the earth appears to depend upon the
sun.
M. You are nearly right, for I know only one process
which does not; that is the ebb and flow of the tide,
which are caused by the attraction of the moon for the
sea when the earth revolves. But that is infinitesimally
small, compared with the work done by the sun.
P. How does it happen that everything depends on
the sun?
M. It happens that the radiation from the sun is the
only source of energy that we have at our disposal. As
everything that takes place can only take place by the
expenditure of work or energy everything depends on
the source of the energy.
P. It does not seem so important to pay attention to
the elements being formed from their compounds and
the compounds being decomposed into elements again
as I had thought. It was such a good scheme.
M. It is less important than the stream of energy
which is poured from the sun on the earth, and is taken
up and stored by plants in order to make life possible.
You can make a picture of this by thinking of a mill.
250 CONVERSATIONS ON CHEMISTRY,
The elements are the wheel which moves in a circle and
continually utilizes the work of the falling water. And
the falling water represents the rays of the sun, without
whose action the mill of life would stop.
Jlff^
M
D2996
Jhemistry.
OOL LIBRARY
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Open Library
American Libraries
TV News
Understanding 9/11